Methods and apparatuses to identify devices

ABSTRACT

An apparatus and method to identify devices including a first set of commands to identify devices in a first state and a second set of commands to identify devices in a second state, wherein devices identified in the first state are placed in the second state and devices identified in the second state are placed in the first state.

FIELD OF THE TECHNOLOGY

The invention relates to the field of devices having an identifier, such as Tags, and further relates to methods and apparatuses for identifying such Tags.

BACKGROUND

Multiple wireless Tags can be interrogated by sending from an interrogating transmitter (e.g., a Reader) a code and having information transmitted by the Tag in response. This is commonly accomplished by having the Tag listen for an interrogation message and for it to respond with a unique serial number and/or other information. The Tags typically have limited power available for transmission data wirelessly to the Reader. It is desirable to extend the range of wireless Tags so that it is not necessary to bring each Tag close to a Reader for reading. However, when the range of the reading system is extended and significant, many Tags will be within the range of the interrogating system so that their replies may corrupt each other.

Current implementations of radio frequency (RF) Tags require considerable logic to handle interface protocol and anti-collision problems which occur when multiple Tags within the range of a Reader attempt to all reply to an interrogating message. For example, current integrated circuits which are used in RF Tags require nearly 3,000 logic gates to handle an interface protocol and to handle anti-collision protocols. This considerable size required by an integrated circuit increases the cost of the RF Tag and thus makes it less likely for such a Tag to be more commonly used. Prior art attempts to avoid collisions when reading multiple RF Tags are described in U.S. Pat. Nos. 5,266,925, 5,883,582 and 6,072,801. However, these prior art approaches provide inefficient solutions for avoiding collision when reading multiple RF Tags.

SUMMARY OF THE DESCRIPTION

Methods and apparatuses to identify Tags are described here. Some of the embodiments of the present invention are summarized in this section.

Embodiments of the present invention includes systems with Readers and Tags in which a Reader queries the Tags with a parameter that specifying a level of probability of reply according to which the Tags individually and randomly decide whether or not to reply. In one example, the Tags can switch between two states: A and B. The query command also specifies a state (A or B) so that only the Tags in the specified state can reply. After successfully sending the Tag identification data from a Tag to the Reader, the Tag switches to the other state from the specified state. In one example, the operations about the two states are symmetric. In one example, the Tags can remember the parameters used in a query to that a short form of query command can be used to repeat the query with the same query parameters.

In one aspect of the present invention, a method to query a plurality of Tags includes: broadcasting a first query command with a first value of a probability parameter where the first value of the probability parameter indicates a first probability of reply according to which each of the plurality of Tags randomly determines whether or not to reply; and detecting a reply in response to the first query command. In one example, in response to a determination that there is no reply to one or more query commands to query according to the first value of the probability parameter, a Reader further broadcasts a second query command with a second value of the probability parameter where the second value of the probability parameter indicates a second probability of reply which is larger than the first probability of reply. In another example, in response to a determination that there is no legible reply to one or more query commands to query according to the first value of the probability parameter due to collision of multiple replies, a Reader further broadcasts a second query command with a second value of the probability parameter where the second value of the probability parameter indicates a second probability of reply which is smaller than the first probability of reply. In one example, the first value is an integer Q; the first probability of reply for one of the plurality of Tags is substantially equal to p^(Q); and, p is less than 1. For example, p can be substantially equal to 0.5. In one example, the first probability of reply for a first one of the plurality of Tags is different from the first probability of reply for a second one of the plurality of Tags. In one example, the first query command further includes a state flag indicating a first state so that Tags in a second state do not reply to the first query command and Tags in the first state reply to the first query command randomly according to the first value of the probability parameter. In one example, a Reader further: 1) broadcasts a second query command with a second value of the probability parameter and a state flag indicating the second state so that Tags in the first state do not reply to the second query command and Tags in the second state reply to the second query command randomly according to the second value of the probability parameter, and 2) detects a reply in response to the second query command. In one example, the first and second query commands are symmetric with respect to the first and second states. In one example, in response to a legible reply to the first query command which reply includes first handshaking data, a Reader further sends a second command including the first handshaking data and receives Tag identification data as a reply to the second command. When the identification Tag data is not received successfully, the Reader further sends a command to indicate an error in receiving the Tag data In one example, a Reader further broadcasts a second query command without specifying a value of the probability parameter to query according to the first value of the probability parameter. The first query command includes second values for a plurality of parameters including the probability parameter; and, the second query command does not specify values for the plurality of parameters to query according to the second values for the plurality of parameters. In one example, the second query command is substantially shorter than the first query command.

In another aspect of the preset invention, a method for a Tag to response to query from a Reader includes: receiving from the Reader a first query command with a first value of a probability parameter, and randomly deciding whether or nor to reply to the first query command so that a probability of reply is according to the first value of the probability parameter. In one example, in response to a random decision to reply, a Tag further sends a reply with first handshaking data, which can be a random number generated in response to the first query command. In one example, the first value is an integer Q; the first probability of reply for one of the plurality of Tags is substantially equal to p^(Q); and, p is less than 1. For example, p can be substantially equal to 0.5. In one example, the first query command further includes a state flag indicating a first state; the Tag do not reply to the first query command if in a second state; and the Tag reply to the first query command randomly according to the first value of the probability parameter if in the first state. In one example, a Tag further: 1) receives a second query command with a second value of the probability parameter and a state flag indicating the second state; and, 2) randomly decides whether or nor to reply to the second query command so that a probability of reply is according to the second value of the probability parameter, if the Tag is in the second state. The Tag does not reply to the second query command if in the first state. In one example, the Tag processes the first and second query command with symmetry with respect to the first and second states. In one example, a Tag further sends a first reply with first handshaking data in response to a random decision to reply, and, in response to receiving from the Reader a second command including the first handshake data, the Tag sends a second reply with Tag identification data. In one example, in response to receiving a query command after sending the second reply, the Tag switches from the first state to the second state; and, after receiving a command indicating an error in receiving the Tag identification data at the Reader, the Tag remains in the first state if a query command is received after the command indicating the error. In one example, a Tag further receives a second query command which does not specify a value of the probability parameter, and the Tag randomly decides whether or nor to reply to the second query command so that a probability of reply is according to the first value of the probability parameter. In one example, the first query command includes second values for a plurality of parameters including the probability parameter; the second query command does not specify values for the plurality of parameters; and, the Tag processes the second query command according to the second values for the plurality of parameters. In one example, the second query command is substantially than the first query command.

The present invention includes methods and apparatuses which perform these methods, including data processing systems which perform these methods, and computer readable media which when executed on data processing systems cause the systems to perform these methods.

Other features of the present invention will be apparent from the accompanying drawings and from the detailed description which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and not limitation in the figures of the accompanying drawings in which like references indicate similar elements.

FIG. 1 shows an example of an identification system which includes a Reader and a plurality of RF Tags.

FIG. 2 shows an example of one embodiment of an RF Tag which may be used with embodiments of the present invention.

FIG. 3 shows an example of an RF Tag according to one embodiment of the present invention.

FIG. 4 illustrates a flowchart representation of a communication method according to one embodiment of the present invention.

FIG. 5 illustrates a flowchart representation of a method for a Tag to communicate with a Reader according to one embodiment of the present invention.

FIG. 6 shows an example of a decision making circuit for a Tag to randomly decide whether or not to reply to a query according to one embodiment of the present invention.

FIG. 7 shows a flowchart representation of a method for a Tag to generate random numbers for communication with a Reader according to one embodiment of the present invention.

FIG. 8 illustrates a flowchart representation of a method for a Reader to read Tag data from a number of Tags according to one embodiment of the present invention.

FIG. 9 illustrates a Tag state diagram according to one embodiment of the present invention.

FIGS. 10-13 illustrate signal modulation for broadcasting from a Reader to Tags according to embodiments of the present invention.

FIGS. 14-17 illustrate signal modulation for a Tag to reply to a Reader to according to embodiments of the present invention.

DETAILED DESCRIPTION

The following description and drawings are illustrative of the invention and are not to be construed as limiting the invention. Numerous specific details are described to provide a thorough understanding of the present invention. However, in certain instances, well known or conventional details are not described in order to avoid obscuring the description of the present invention. References to one or an embodiment in the present disclosure are not necessarily references to the same embodiment; and, such references mean at least one.

FIG. 1 illustrates an example of an identification system 100 which includes a Reader 101 and a plurality of Tags 131, 133, 135, . . . , and 139. The system is typically a Reader-talks-first RF ID system using either passive or semi-passive active backscatter transponders as Tags. The incorporation of a battery and/or memory into a Tag is an expanded feature to facilitate longer read range; however, the use of the battery does require certain trade-offs, such as higher costs, limited longevity, larger form factor, greater weight, and end-of-life disposal requirements. Thus, the Tags 131-139 may have memory and/or a battery or may have neither of these elements. It will be appreciated that different types of Tags may be mixed in a system where a Reader is interrogating Tags with batteries and Tags without batteries. There are at least 4 classes of Tags which may be used with the present invention: (1) no power source on the Tag except for power which is obtained from the Tag's antenna, but the Tag does include a read-only memory which has the Tag's identification code; (2) a Tag without internal power, but when powered from the Reader, can write data to non-volatile memory in the Tag; this type of Tag also includes memory for storing the identification code; (3) a Tag with a small battery to provide power to the circuitry in the Tag. Such a Tag may also include non-volatile memory as well as memory for storing the Tag's identification code; (4) a Tag which can communicate with each other or other devices.

FIG. 1 shows an embodiment of a Reader. The Reader 101 typically includes a receiver 119 and a transmitter 123, each of which are coupled to an I/O (input/output) controller 117. The receiver 119 may have its own antenna 121, and the transmitter 123 may have its own antenna 125. It will be appreciated by those in the art that the transmitter 123 and the receiver 119 may share the same antenna provided that there is a receive/transmit switch which controls the signal present on the antenna and which isolates the receiver and transmitter from each other. The receiver 119 and the transmitter 123 may be similar to conventional receiver and transmitter units found in current Readers. The receiver and transmitter typically operate, in North America, in a frequency range of about 900 megahertz. Each is coupled to the I/O controller 117 which controls the receipt of data from the receiver and the transmission of data, such as commands, from the transmitter 123. The I/O controller is coupled to a bus 115 which is in turn coupled to a microprocessor 113 and a memory 111. There are various different possible implementations which may be used in the Reader 101 for the processing system represented by elements 117, 115, 113, and 111. In one implementation, the microprocessor 113 is a programmable microcontroller, such as an 8051 microcontroller or other well-known microcontrollers or microprocessors (e.g. a PowerPC microprocessor) and the memory 111 includes dynamic random access memory and a memory controller which controls the operation of the memory, memory 111 may also include a non-volatile read only memory for storing data and software programs. The memory 111 typically contains a program which controls the operation of the microprocessor 113 and also contains data used during the processing of Tags as in the interrogation of Tags. In one embodiment further described below, the memory 111 would typically include a computer program which causes the microprocessor 113 to send search commands through the I/O controller to the transmitter and to receive responses from the Tags through the receiver 119 and through the I/O controller 117. The Reader 101 may also include a network interface, such as an Ethernet interface, which allows the Reader to communicate to other processing systems through a network. The network interface would typically be coupled to the bus 115 so that it can receive data, such as the list of Tags identified in an interrogation from either the microprocessor 113 or from the memory 111.

FIG. 2 shows an example of one implementation of a Tag which may be used with the present invention. The Tag 200 includes an antenna 201 which is coupled to a receive/transmit switch 203. This switch is coupled to the receiver and demodulator 205 and to the transmitter and modulator 209. A correlator and controller unit 207 is coupled to the receiver and demodulator 205 and to the transmitter 209. The particular example shown in FIG. 2 of a Tag may be used in various embodiments in which a memory for maintaining data between commands is maintained in the Tag and in which a bit by bit correlation occurs in the Tag. The receiver and demodulator 205 receives signals through the antenna 201 and the switch 203 and demodulates the signals and provides these signals to the correlator and controller unit 207. Commands received by the receiver 205 are passed to the controller of the unit 207 in order to control the operation of the Tag. Data received by the receiver 205 is also passed to the control unit 207, and this data may include parameters for a query command and handshake data from a handshake command in the embodiments described below. The transmitter 209, under control of the control unit 207, transmits responses or other data through the switch 203 and the antenna 201 to the Reader. It will be appreciated by those in the art that the transmitter may be merely a switch or other device which modulates reflections from an antenna, such as antenna 201.

In one embodiment of the present invention, to achieve Tag cost low enough to enable ubiquitous use of Tags in the supply chain, the Tags are designed with properties, such as small Integrated Circuit (IC) area to permit low cost, small memory, precise timing not required, atomic transactions to minimize Tag state storage requirements and others. Such Tags can be produced at low cost. However, other Tag designs can also be used. Further, it is understood that the method of avoid collisions in communications according to embodiments of the present invention can also be used in other similar situations.

FIG. 3 shows an example of an RF Tag according to one embodiment of the present invention. In one embodiment, a VLC (Very Low Cost) Tag 300 includes an antenna 301 and an integrated circuit 303, connected together. The Tag IC 303 implements the command protocol and contains the ePC (Electronic Product Code). The antenna 301 receives the Reader interrogation signals and reflects the interrogation signal back to the Reader in response to a modulation signal created by the IC 303. The Tag IC 303 implements the VLC Tag by combining an RF interface and power supply 311, data detector and timing circuit 313, command and control 315, data modulator 317 and memory 319. In one embodiment, command and control 315 includes static logic which implements the communication protocol according to embodiments of the present invention.

The RF Interface and Power Supply 311 converts the RF energy into the DC power required for the Tag IC 303 to operate, and provides modulation information to the Data Detector and Timing circuit 313. The RF interface also provides a means of coupling the Tag modulation signals to the antenna for transmission to the Reader. The Data Detector and Timing circuit 313 de-modulates the Reader signals and generates timing and data signals used by the command and control 315. The command and control 315 coordinates all of the functions of the Tag IC 303. The command and control 315 may include state logic to interpret data from the Reader, perform the required internal operations and determines if the Tag will respond to the Reader. The command and control 315 implements the state diagram and communications protocol according to embodiments of the present invention. The memory 319 contains the ePC code of the item Tagged by a VLC Tag. The data modulator 317 translates the binary Tag data into a signal that is then applied to the RF Interface 311 and then transmitted to the Reader (e.g., Reader 101).

The design and implementation of the Tags can be characterized in layers. For example, a physical and environmental layer characterizes the mechanical, environmental, reliability and manufacturing aspects of a Tag; a RF transport layer characterizes RF coupling between Reader and Tag; and, a communication layer characterizes communications/data protocols between Readers and Tags. Various different implementations of Tags at different layers can be used with embodiments of the present invention. It is understood that the implementations of the Tags are not limited to the examples shown in this description. Different Tags or communication devices can use methods of the embodiments of the present invention for communication according to the needs of the target application.

In one embodiment of the invention, a Tag may be fabricated through a fluidic self-assembly process. For example, an integrated circuit may be fabricated with a plurality of other integrated circuits in a semiconductor wafer. The integrated circuit will include, if possible, all the necessary logic of a particular RF Tag, excluding the antenna 301. Thus, all the logic shown in the Tag 300 would be included on a single integrated circuit and fabricated with similar integrated circuits on a single semiconductor wafer. Each circuit would be programmed with a unique identification code and then the wafer would be processed to remove each integrated circuit from the wafer to create blocks which are suspended in a fluid. The fluid is then dispersed over a substrate, such as a flexible substrate, to create separate RF Tags. Receptor regions in the substrate would receive at least one integrated circuit, which then can be connected with an antenna on the substrate to form an RF Tag. An example of fluidic self-assembly is described in U.S. Pat. No. 5,545,291.

FIG. 4 illustrates a flowchart representation of a communication method according to one embodiment of the present invention. A Reader broadcasts a query command with a specified value of Q parameter to Tags in state A (401). In response to the query command, each of the Tags in state A individually and randomly decides whether or not to reply to the query command such that a probability of replying is in accordance with the value of the Q parameter (403). Tags in state B do not reply to the query command which is for tags in state A. The Reader then detects any reply to the query command (405). It is determined whether there are too few replies (407). For example, when the Reader obtains no reply for a number of query commands with the specified value of Q parameter, the Reader may determine that the specified level of probability to reply is too low and there are too few replies. When there are too few replies, the Reader adjusts the value of Q parameter to increase the probability to reply (411). Similarly, it is determined whether there are too many replies (409). When there are too many replies, the replies from different Tags corrupt each other. Thus, the Reader adjusts the value of Q parameter to decrease the probability to reply when there are too many replies. If no legible reply is received (417), the Reader broadcasts a query command without specifying parameters so that the previously transmitted parameters are used for the current query (415). Since the same parameters for the query are not transmitted again, it is faster to issue the query command to repeat the previous query than to issue the query command with all the parameters. In response to the new query command, each of the Tags in state A then individually and randomly decides whether or not to reply to the query command such that a probability of replying is in accordance with the value of the Q parameter (403).

When the value of Q parameter is adjusted to a suitable value, the probability of obtaining one legible reply from a large number of Tags will be high. Thus, the Reader can simply repeat the previous query without adjusting query parameters until there are too few (or too many) replies.

When one legible reply is received (417), the Reader communicates with the Tag which provides the reply (419). In one embodiment of the present invention, the reply from the Tag includes data that identifies the Tag so that the Reader can address to the Tag that provides the legible reply. In one embodiment, a Tag generates a random number for the purpose of handshaking with the Reader. During the communication with the Tag, the Reader obtains Tag Identification data from the Tag. If the communication with the Tag is successful (421), the Tag switches from state A into state B (423); otherwise, the Tag remains in state A (425). Once the Tag is in state B, the Tag does not response to the query for Tags in state A. Thus, the Reader can communicate with the Tags in state A one at a time until all Tags are in state B.

In one embodiment of the present invention, the operations with respect to state A and state B are symmetric. For example, the Reader can broadcast a query command with a specified value of Q parameter to Tags in state B. In response to the query command for Tags in state B, each of the Tags in state B individually and randomly decides whether or not to reply to the query command such that a probability of replying is in accordance with the value of the Q parameter. Tags in state A do not response to the query for Tags in state B. If the communication with the Tag in state B is successful, the Tag switches from state B into state A; otherwise, the Tag remains in state B. Thus, the Reader can sort the Tags from state A into state B one at a time, or sort the Tags from state B into state A one at a time.

Alternatively, the operations with respect to state A and state B may be not symmetric. For example, the Reader can sort the Tags from state A into state B one at a time but not from state B into state A one at a time. In such an implementation, the Reader can first place the Tags into state A before starting to read Tag data from Tags one at a time.

FIG. 5 illustrates a flowchart representation of a method for a Tag to communicate with a Reader according to one embodiment of the present invention. In operation 501, the Tag receives commands from a Reader. After receiving a query command with a query parameter Q for Tags in state A (e.g., QueryA) (503), the Tag determines if it is in state A (507). If the Tag is not in state A, the Tag does not reply to the query for Tags in state A.

Similarly, after receiving a query command with a query parameter Q for Tags in state B (e.g., QueryB) (505), the Tag determines if it is in state B (507). If the Tag is not in state B, the Tag does not reply to the query for Tags in state B.

If the query matches the state of the Tag (e.g., the Tag in state A receives a query for Tags in state A or the Tag in state B receives a query for Tags in state B), the Tag randomly determines whether or not to reply to the query command such that a probability of replying is in accordance with the query parameter (e.g., having a probability of 0.5^(Q) to reply). If the Tag decides to reply (513), the Tag replies to the query command with handshake data (e.g., a random number).

When the Tag receives a query command without parameters (e.g., QueryRep) (517), it is determined whether the Tag obtained query parameters from a previous query command (519). If the Tag has the query parameters from a previous query command (e.g., a previous QueryA or QueryB command), the Tag responses to the query using the same parameters that were used for the previous query command (521). For example, if the previous query command is for Tags in state A, the current query command without parameters is also for Tags in state A. Thus, operation 507 is performed to check if the query is intended for the Tag. Similarly, if the previous query command is for Tags in state B, the current query command without parameters is also for Tags in state B so that operation 509 is performed. The Q parameter used in processing the previous query command is also used for the processing of the current query command without parameters. In one embodiment of the present invention, when a suitable value of Q parameter is reached, the Reader issues many query commands without parameters to repeat the query of the same parameters. Since the query command without parameters is quick to transmit (and quick to process), the time to process a large number of Tags can be shortened using such a query command without parameters.

When the Tag receives a command to handshake with handshake data from the Reader (e.g., Ack) (523), the Tag checks if the received handshake data matches the handshake data sent from the Tag (525). If the handshake data do not match (527) (e.g., the handshake command is not in response to a reply sent from the Tag or the handshake data received from the Reader is different from the handshake data sent from the Tag), the Tag does not reply. Otherwise, the Tag sends Tag data (e.g., ePC) to the Reader (529) and enters into “waiting to change state” (531). In one embodiment, the Tag assumes that the Reader receives the Tag data unless the Reader transmits a command to indicate that the Tag data is not received. For example, when the Tag receives a command to prevent state change (e.g., NAk) (533), the Tag exits “waiting to change state” (537). When the Tag receives a command other than to handshake or to prevent state change (e.g., receiving QueryA, QueryB or QueryRep) (539), the Tag changes Tag state (e.g., from State A to State B, or from State B to State A) (543) if the Tag is waiting to change state (541). In another embodiment, the Tag always assumes that the Reader receives the Tag data The Tag changes its state from A to B, or from B to A, if a query command is received while it is waiting to change state after sending the Tag data It is understood that operations 541 and 543 are performed before operation 507 or 509 is performed. Thus, after replying to a query for tags in state A and sending the Tag data, the tag in state A switches into state B and does not reply to a further query for tags in state A. To prevent the Tag from changing state, the Reader can broadcast a command to prevent state change (e.g., NAk) before another query command.

FIG. 6 shows an example of a decision making circuit for a Tag to randomly decide whether or not to reply to a query according to one embodiment of the present invention. A random bit generator (601) generates one bit of random information at a time. A number of random bits are stored in memory 603. For example, when a new bit of random information is generated, it is shifted into the memory so that the first bit in the memory contains the new bit of random information and the oldest bit of random information is discarded. When the Tag received the Q parameter from the Reader (e.g., in a QueryA command or a QueryB command), the value of the Q parameter is stored in memory 607. A logic circuit (650) determines if the first Q bits (e.g., the most recent Q bits) in memory 603 are all zeros. If the first Q bits in memory 603 are all zeros, the Tag decides to reply to the query. Otherwise, the Tag does not reply. When Q is zero, the Tag always decides to reply if the Tag is in the specified state.

In one embodiment, the random bit generator (601) has a probability of (½) to generate zeros. Thus, for a given Q value, the probability to reply is (½)^(Q). The random bit generator (601) may generate random bits at a rate of one bit per command, or faster than one bit per command, or slightly slower than one bit per command. It is understood that different Tags may generate the random bits at different rates. Further, the random bit generator (601) may not generate zeros with a probability of (½). For example, the important Tags may be biased to have a probability larger than ½ to generate zeros. Thus, these Tags are more likely to satisfy requirement that the first Q bits are all zeros. As a result, these Tags have a larger probability to reply earlier than other Tags.

From the above example, it is understood that the Tag can randomly decide to reply with a probability of replying controlled by the Q parameter. Different implementations can be used to achieve such controlled random decision-making. For example, it may request that the oldest Q bits in the memory are all ones. Since adjusting the value of the Q parameter can adjust the probability of replying, a Reader can adaptively adjust the Q value to increase the probability of getting a single legible reply from a large number of Tags that are in the range.

FIG. 7 shows a flowchart representation of a method for a Tag to generate random numbers for communication with a Reader according to one embodiment of the present invention. Operation 701 generates a random bit (e.g., using a random bit generator 601). It is then determined whether the Tag has finished handshaking with a Reader (703). If the Tag is in the process of handshaking with the Reader, the random bit is not used to update the information in the memory (e.g. 603). Thus, the random number in the memory remains the same during the process of handshaking. In the process of handshaking, the Tag sends the content of the memory of random bits (e.g., 16-bit memory) to the Reader as the handshake data and receives a handshake command (e.g., Ack) with handshake data back from the Reader. If the handshake data received from the Reader matches the handshake data sent from and maintained at the Tag, handshaking is successful and the Tag can send the Tag data to the Reader in response. If the Reader does not send the handshake command again (or the handshake data does not match), the Tag finishes handshaking with the Reader (e.g., by sending another query command). When the Tag is not in handshaking with the Reader, the Tag does not need to freeze the content of the memory of random bits. Thus, the Tag shifts the random bit into the memory of random bits (705) to update the content. Based on this description, a person skilled in the art can envision various alternative implementations. For example, the random bit may be generated only in response to a query command.

In one embodiment of the present invention, the entire content of the memory of random bits (e.g., 603) that is used for making the random decision is used as the handshake data Alternatively, only a portion of it may be used as the handshake data For example, when the Tag replies if the first Q bits are all zeros, the Tag may use only the last (16-Q) bits of the random bit memory as handshake data Alternatively, the Tag may use other random numbers as the handshake data.

FIG. 8 illustrates a flowchart representation of a method for a Reader to read Tag data from a number of Tags according to one embodiment of the present invention. After broadcasting a query command with a Q parameter for Tags in state A (801), a Reader detects any reply from the Tags with handshake data (803). When there is no reply (805), it is determined whether the Q parameter is already equal to zero. If the Q parameter is equal zero and no reply is received in response to the query command, it can be determined that there is no Tag in state A within the range, since any Tag in state A receiving the query command will reply when the Q parameter is equal to zero. If the Q parameter is not already zero, the Reader can reduce the Q parameter to increase the probability of receiving a reply. For example, the Reader can maintain a parameter Q_(f) as a floating point number so that Q is determined from Int(Q_(f)). The Reader can update Q_(f) as Min(Q_(f)/1.4, 0.9) and update Q as Int(Q) (811, 815), when there is no reply. When there are multiple replies from different Tags that corrupt each other, the Reader cannot obtain legible handshake data from the replies (817). To avoid collision, the Reader can increase the Q parameter to decrease the probability of receiving multiple replies. For example, the Reader can update Q_(f) as Q_(f)×1.4 and update Q as Int(Q) (813, 815) when multiple replies collide to corrupt each other.

Note that when Reader can obtain legible handshake data from one reply, the Reader does not have to increase the Q parameter even if there is collision. For example, when a weak reply collides with a strong reply, the Reader can still obtain the handshake data from the strong reply. In this case, the Reader can simply ignore the weak reply and start to handshake with the Tag that sends the strong reply. Thus, hidden collisions improve performance, since weak Tags are protected by the ACK handshake and the stronger Tag is still counted if the Reader can extract its handshake.

After legible handshake data is received as reply to the query command (817), the Reader handshakes with the Tag that sends the handshake data (e.g., by broadcasting a command, such as Ack, with the handshake data). Then, the Reader tries to receive Tag data (e.g., Tag identification data, such as ePC) from the Tag (821). For example, if the Tag determines that the handshake data in the Ack command matches the handshake data sent from the Tag, the Tag transmits the Tag identification data as a reply to the Ack command. If the Tag receives legible Tag data (823), the Tag can broadcast a command to repeat the previous query command without re-broadcasting the parameters for the query (829). In response to the query command, the Tag that just sent the Tag data switches from state A to state B so that it does not response to the query for Tags in state A. The Tags in state A use the previous query parameters for the current query. However, if the Tag data is not legible (823), the Reader may try again to handshake with the Tag (819) or broadcast a command to indicate that the Tag data is not received (827).

In one embodiment of the present invention, the Tag switches state in response to any query commands after transmitting the Tag data. Thus, after receiving the legible Tag data, the Reader can choose to broadcast a command to repeat the previous query or to broadcast a query command with new query parameters. Alternatively, the Tag can be implemented such that it switches state, after transmitting the Tag data, only in response to the command that repeats the previous query command (e.g., QueryRep). Thus, the Reader can use one QueryRep command to cause: 1) the Tag that has just sent the Tag data to switch state to leave the set of Tags to be interrogated; and, 2) other Tags to be queried and to make random decision about whether or not to reply to the query.

In one implementation of the present invention, system communication follows a two-stage command-reply pattern where the Reader initiates the transaction (Reader Talks First, RTF). In the first phase, the Reader provides power to one or more passive Tags with continuous wave (CW) RF energy. Tags power up, ready to process commands after one command which is used for synchronization of its clocks. The Reader transmits information to the field by amplitude modulation using the Reader-to-Tag encoding scheme described below. On completion of the transmission, the Reader ceases modulation and maintains RF to power the Tags during the reply phase. Tags communicate with the Reader via backscatter modulation during this period using the four (4)-phase bit encoding scheme described below.

In one implementation, basic commands are designed to limit the amount of state information the Tags have to store between transactions. The power available to a passive Tag is a complicated function of transmitted power, Tag/Reader antenna orientations, local environment and external sources of interference. Tags on the margin of the RF field are powered unreliably and therefore cannot be counted on to maintain a memory of previous transactions with the Reader. In particular, moving Tags or objects may cause the Tag to have power only for brief intervals, primarily due to multi-path interference. In one implementation, it is designed to allow the efficient counting of Tags under these conditions by minimizing the total transaction time, and by allowing rapid recovery from missed commands. Tags which have power on threshold and receiving three commands (e.g., a prior command to spin up on, a query, and an ACK with its reply) in as little as 3 milliseconds can be inventoried.

In one implementation, there is only one bit of state for each session, between command groups, and the impact of that state is further lessened by symmetrizing the command set about those two states, as described below.

In one implementation, each Tag has four sessions available, each with a single bit of independent state memory. The backscatter mode and relative rate are the same for all of the sessions, and the random reply register is the same for all sessions. The state of being selected is also the same for all sessions. This session structure allows up to four Readers or processes to communicate with the Tag population in a multitasking environment, but they can do so with a complete command group. A command group starts with a QueryA/B (QueryRep does not start a command group), and continues through a ACK, and ends with either the command after the ACK (which completes the transaction from the Tags point of view), or at the end of the use of the SELECTED state by a process.

One example of the use of two sessions is a portal Reader which is counting all Tags coming through a portal, but wants to preferentially count pallets. It could then run two simultaneous processes on the Tag population. Session 0, for example, could be used by one process to sweep the entire population of Tags between (session 0) state A and state B to ensure counting all Tags that it touches once, regardless of their former state. Session 1 could selectively mask all pallet Tags to state A for that session and all other Tags to state B, and count them preferentially in an interleaved process, without interfering with the ongoing inventory in the first process.

A similar example would be a set of store inventory Readers. They would be set to synchronize their inventory type, for example all inventory Readers using session 0 at the Tags to inventory from the A state to the B state for a ten second interval, and then inventorying from the B state back to the A state. This ensures that all Tags are counted by one inventory Reader once per cycle. Simultaneously, a handheld Reader could use session 1 to look for a specific ePC by masking a sufficient portion of that ePC to state A while masking all other Tags to state B. It then uses session 1 QueryA commands to seek that Tag or Tag type. This avoids interference with the store inventory Readers as long as command groups do not collide (interleave) and as long as RF interference is avoided.

In one implementation, each Tag has four sessions available, each with a single bit of independent state memory (A/B). The backscatter mode and rate are the same for all of the states, and the random reply register is the same for all sessions. The state of being selected is also the same for all sessions.

FIG. 9 illustrates a Tag state diagram according to one embodiment of the present invention. The Tag state diagram shows the states that a Tag may take within command groups. A command group is a set of sequential commands which start with a QueryA/B command and ends with the Tag leaving the selected or acknowledge state. Between Tag groups, the state in each session is A or B. The DEAD state is a permanent state. For session 0, states A and B are persistent states, even in the absence of power. After a long period of time, at least 1 second but possibly hours, state B reverts to state A. In state A, all Tags respond to QueryA commands but not QueryB commands. In state B, QueryB commands are responded to, but not QueryA commands. If the state memory has expired for a particular session, the Tag enters into state A upon power up for that session.

Sessions other than the 0 session may not have a persistent A-B flag, and may remember their state only while power is available. If their state is lost, the state for that session reverts to state A.

Upon power up, the Tag resets all of its internal state except for the state flags for each of the 4 sessions. The Tag syncs its clock to the rising edges of the sync bits provided by the first command, but is not allowed to act upon the first command after power up. It maintains its clock with sufficient accuracy to decode the next command. If clock frequency needs to be corrected by more than 20% during any command spinup, the Tag does not respond to that command but waits until the next QueryA or QueryB command. This is designed to prevent inappropriate and possibly interfering responses.

For each session, while a Tag is in state A, it responds to QueryA commands but not QueryB commands. A Tag enters into state A upon the appropriate SetState command, and upon successful completion of an inventory handshake starting with a QueryB.

For each session, while a Tag is in state B, it responds to QueryB commands but not QueryA commands. A Tag enters into state B upon the appropriate SetState commands, and upon successful completion of an inventory handshake started from a QueryA. A Tag may remain in state B for session zero (0) even with loss of power of up to 1 second.

For each independent session, if the state memory for state B is lost, the Tag powers up in state A.

The Dead State is a permanent change in the state of the Tag state entered on receipt of a valid Kill Command and Kill Code sequence. The Dead state is implemented by a permanent change in the physical Tag, such as programming E² or blowing a fuse.

Some embodiments of the present invention relates to RFID protocol for achieving a fast, robust, compatible and extensible protocol for class I, class II, and class III RFID Tags, while allowing very inexpensive implementations for both Tags and Readers. In one embodiment, the RF transport layer focuses on UHF operations; for example, the protocol can be applicable from 400 MHz to 2.45 GHz.

There are at least four classes of Tags for different applications and cost requirements. Tags of different classes that meet the requirements of all of the design layers are able to work together. Tags may also include a standardized wired I/O interface to sensors, clocks, displays and other devices.

Class I Tags are simple passive, read-only backscatter Tags, designed for lowest cost production. Class I Tags have a one-time programmable memory, write once identity memory, 64 or 96 bit ePC Code, and optional recycling code.

Class II Tags are passive backscatter Tags with higher functionality and cost than Class I. In addition to the Class I features, Class II Tags: can write and read data into and out of Tags, have read-write memory, may have battery power for non-communication purposes, and optionally have sensors and data loggers.

Class-III Tags semi-passive backscatter Tags. In addition to class II features, class III Tags have built-in battery or other energy source to support increased read range.

Class IV Tags are modem-like semi-passive or active (transmitter) Tags that can wirelessly communicate with each other and/or other devices. Class IV may also emulate the class III Tags in communication with Readers.

RFID systems according to one implementation of the present invention have features includes: identification of single Tags in the field of a Reader, anti-collision functionality to manage reading of multiple Tags in the field of a Reader, errors management in the system from sources of RF interference and marginal Tags, operations in compliance with local RF regulatory requirements, and coexistence with systems operating in compliance with local RF regulatory requirements.

Some detailed examples are provides below. However, according to this description, different detailed design and implementation can be envisioned by those skilled in the art. Overall system architecture is typically a function of the marketplace.

In the following description, references to bits in the memory of RFID Tags are made. When referring to bits in Tag memory, the words “up” or “higher” generally refer to the Most Significant Bit (MSB) direction and the words “down” or “lower” to the Least Significant Bit (LSB) direction. For example, the binary rerepresentation of the decimal number seven (7) is 0111. Shifting each bit “up” or “higher” by one yields the decimal number fourteen (14), represented in binary as, 1110.

In one embodiment of the present invention, the command format is designed around the notion that to support very low cost (VLC) Tags. The Reader performs extra work, allowing the Tags to be as simple and inexpensive as possible. This falls largely into two areas, dealing with timing uncertainty and dealing with limited long term memory in the Tags. However, other types of Tags (e.g., Class m and higher Tags, such as battery powered devices with large memories) may also support these command formats (e.g., for compatibility reasons).

VLC, single-chip Tags typically have limited oscillator stability, since quartz crystals are cost and size prohibitive. In one embodiment, the Tags use the bit timing of commands to synchronize their internal clock, and need to be powered up and see one full command packet before they can start decoding further commands. Replies from these Tags are structured such that the Reader can interpret the information transmitted by the Tags at whatever clock rate the Tag is able to provide. This scheme is similar in concept to auto-synchronization schemes used in magnetic card or barcode Readers.

In one implementation, three classes of commands are provided, including basic commands, programming commands and data link commands. Basic commands provide object identification, sorting, inventory, etc. Programming commands support Tag data initialization and programming by the Tag manufacturers prior to Tagged items entering the supply chain. Data link commands provide a data link layer for class II and higher Tags.

One embodiment of the present invention uses Huffman encoded commands, for example, 18 bits for Query A/B (with 4 bit Q), 6 bits for QueryRep (last query repeat), 23 bits for ACK (including 16 bits of data), 13 bits for NAK (used Rarely, e.g., on data errors) and 13 bits or more for parameters of other commands. Thus, a QueryRep command is substantially shorter than a QueryA or a QueryB command.

Detailed examples of command structure in one implementation are described below. In the following examples of command structures, the command fields are generally listed in the order in which they are transmitted.

In one implementation, there are three types of inventory commands, query, ACK, and NAK. A Query command starts a transaction, to which one or more Tags respond with a random 16-bit number. If the Reader successfully extracts a 16-bit number, it is sent back to the Tag for handshaking through an ACK command. A Tag only responds if the 16-bit number sent by the ACK command matches the number that the Tag sent. A Tag which has its 16-bit random number confirmed then replies with a prefix, its CRC (Cyclic Redundancy Check), and its ePC (Electronic Product Code). The Tag then transitions its internal state for that session from A to B (or from B to A) unless it gets a NAK. If it receives a NAK, it stays in the former state.

In one implementation, the Reader transmits the SPINUP bits first. Within each field, the LSB (Least Significant Bit) is transmitted first. Every command is preceded by four Manchester zero bits to enable clock spinup. The clock spinup bits are followed by a Manchester high violation, and the command bits, and parameters which vary from command to command. If the Tag clock setting mechanism is required to adjust the clock by more than 20%, or if the Tag does not see the spinup bits or Manchester high violation the Tag ignores the command except for the purpose of improving its clock sync. If any command does not match the data pattern of a valid command, the Tag does not change its internal state, and does not modulate its backscatter. At the start of every command, the Tag refreshes the state memory for each of its four sessions. If the Tag is starting up from a power on reset, it enters the “Awaiting first query” state.

When the Tag is reset through “Power On”, the Tag always enters into the state of “Awaiting first query”.

When the clock is not synced, or the clock changed by more than 20%, or spinups or Manchester violation have not been seen, or bad command bits or bad CRC data is received, a bad command is received at the Tag. When the starting state condition is in “Awaiting first query”, “Ready”, or “Selected”, the Tag remains in the same state condition in response to a bad command. When the starting state condition is “Waiting for ACK” or “Acknowledged”, a bad command causes the Tag to enter into the state of “Ready”.

A QueryA command has parameters which include a session number, backscatter mode, and the relative Tag to Reader data rate. They have a data payload which is a number Q. When a Tag receives the QueryA command, if it is in state A for that session, it responds with a probability of (½)^(Q). The Tag responds to each query with an independent (½)^(Q) probability. The reply from the Tag contains 16 random bits which the Tag also remembers until the next command.

For example, a QueryA command includes spinup bits (4-bit, “0000”), Manchester high violation (1-bit), Command bits (1-bit, “0”), Session number [S] (2-bit), A/B flag (1-bit, “0” for QueryA and “1” for QueryB), Backscatter mode [M] (2-bit, e.g., “00” for FM0, “01” for F2F), Backscatter relative rate [R] (2-bit), and Q parameter [Q] (4-bit). Masking can be used for populations larger than 120,000.

In response to a QueryA command, a Tag: 1) sets session number to [S]; 2) sets state flag for query to “A”; 3) sets the Q parameter to [Q]; 4) sets the Backscatter mode to [M]; and, 5) sets the Backscatter Rate to [R]. The Tag computes a random number and makes random decision according to [Q]. In addition, if the Tag is in the starting state of “Selected” or “Acknowledged”, the Tag switches to state B if it is in state A and to state A if it is in state B. Then, if the Tag is in state A and the random decision is positive, the Tag replies with the random number and enters into the state of “Waiting for ACK”; otherwise, the Tag enters into the state of “Ready”.

A QueryB command is symmetric to a QueryA command with respect to state A and B. A successful completion of a handshake cycle after a QueryB command places the Tag into state B for that session. The Tag reply to a QueryB command is in the same format as to a QueryA command.

For example, a QueryB command includes spinup bits (4-bit, “0000”), Manchester high violation (1-bit), Command bits (1-bit, “0”), Session number [S] (2-bit), A/B flag (1-bit, “0” for QueryA and “1” for QueryB), Backscatter mode [M] (2-bit, e.g., “00” for FM0, “01,” for F2F), Backscatter relative rate [R] (2-bit), and Q parameter [Q] (4-bit). Masking can be used for populations larger than 120,000.

In response to a QueryB command, a Tag: 1) sets session number to [S]; 2) sets state flag for query to “B”; 3) sets the Q parameter to [Q]; 4) sets the Backscatter mode to [M]; and, 5) sets the Backscatter Rate to [R]. The Tag computes a random number and makes random decision according to [Q]. In addition, if the Tag is in the starting state of “Selected” or “Acknowledged”, the Tag switches to state B if it is in state A and to state A if it is in state B. Then, if the Tag is in state B and the random decision is positive, the Tag replies with the random number and enters into the state of “Waiting for ACK”; otherwise, the Tag enters into the state of “Ready”.

A QueryRep command repeats the last query with the same parameters. If the Tag has not seen a QueryA/B since power on reset, it does not respond to a QueryRep. This command is typically the most common command except in the complete atomic mode. The Tag reply to a QueryRep is in the same format as that for a QueryA or QueryB command.

For example, a QueryRep command includes spinup bits (4-bit, “0000”), Manchester high violation (1-bit), and, Command bits (1-bit, “0”).

In response to a QueryRep command, a Tag in the state of “Awaiting first query” does not reply and remains in the state of “Awaiting first query”. A Tag in the state of “Acknowledged” or “Selected” does not reply, switches to state B if it is in state A and to state A if it is in state B, and enters into the state of “Ready”. A Tag in the state of “Ready” or “Selected”: 1) computes a random number and makes random decision according to [Q]; 2) checks if the state of Tag matches the state flag for query (e.g., state flag for query is “A” while the Tag is in state A or state flag for query is “B” while the Tag is in state B); and, 3) if the state of Tag matches the state flag for query and the random decision is positive, replies with the random number and enters into the state of “Waiting for ACK”. If the state of Tag does not match the state flag for query or the random decision is negative, the Tag does not reply and enters into the state of “Ready”.

A reply from a Tag in response to a query command (e.g., QueryA, QueryB or QueryRep) contains 16-bit handshake data. Tags in the currently set backscatter mode, and at the currently set backscatter rate. (Both are set by a QueryA or QueryB command). They send a one, followed by a crowbar off for one bit period, 16 bits of random data, followed by another crowbar off period, and a trailing 1.

For example, a reply from a Tag in response to a query command includes TAGSPINUP (1-bit, “1∞), High violation (Crowbar off for FM0 or F2F bit period), Tag handshake (16-bit, Random data), High violation (Crowbar off for FM0 or F2F bit period), and TAGTRAILER (1-bit, “1”).

An ACK command is sent by the Reader when if successfully extracts the handshake from a Tag reply. The ACK follows immediately after a Query command, without intervening commands except other ACKs. It also follows within T_(coast), together considered as an atomic “command” of the protocol. If the ACK command is received by the Tag does not contain the handshake that it replied to the immediately preceding Query, it does not reply.

For example, an ACK command includes spinup bits (4-bit, “0000”), Manchester high violation (1-bit), command bits (2-bit, “10”) and Handle data (16-bit, the data that was sent to the Reader in the immediately preceding query).

In response to an ACK command, a Tag in the starting state of “Awaiting first query” does not reply and remains in the state of “Awaiting first query”. A Tag in the starting state of “Ready” or “Selected” does not reply and enters into the state of “Ready”. A Tag in the starting state of “Waiting for ACK” or “Acknowledged” checks if the handle data in the ACK command matches the random number sent to the Reader in the immediately preceding query. If there is a match, the Tag scrolls back ePC and CRC as a reply and enters into the state of “Acknowledged”; otherwise, the Tag enters into the state of “Ready”.

A reply in response to an ACK command contains ePC and CRC. Tags matching their 16-bit handshake to that sent by the Reader reply in the currently set backscatter mode, and at the currently set backscatter rate. (Both are set by a QueryA or QueryB command). They respond by sending four (4) one (1) bits followed by a high violation for one R-T bit time, and all the identification data in the Tag starting at bit 0. Data sent by the Reader to the Tag may be of variable length. The data is followed by a crowbar off (high) violation, and 4 trailing ones (1)s.

For example, a reply from a Tag in response to an ACK command includes TAGSPINUP (4-bit, “1111”), High violation (Crowbar off for FM0 or F2F bit period), Tag data (variable size, ePC, CRC, recycling data, etc) High violation, and TAGTRAILER (4-bit, “1111”).

If a Reader does not receive a response to an ACK, the Reader transmits a NAK. If it receives a garbled response to an ACK it may transmit a NAK or try an ACK again. The NAK (or any command other than a Query, a repeat ACK, or select) is used to signal a Tag that it has not been recorded, and should stay in its former (A or B) state.

In a query-ACK inventory, it is used only upon data error. The NAK command also ends the SELECTED state. There is no reply to a NAK.

For example, a NAK command includes spinup bits (4-bit, “0000”), Manchester high violation (1-bit), and command bits (8-bit, “11000000”).

In response to a NAC command, a Tag in the starting state of “Awaiting first query” does not reply and remains in the state of “Awaiting first query”. A Tag in the starting state of “Ready”, “Selected”, “Waiting for ACK”, or “Acknowledged” does not reply and enters into the state of “Ready”.

A SetState command is used to restrict searches, for direct addressing and masking, including various set operations (e.g., union). Set operations are performed by using SetState commands. Masking would start by picking an unused session, and then setting all of the Tags to the desired state in that session by issuing a string of SetState commands. Tags whose state is changed respond with the ScrollID preamble, enabling applications which maintain an inventory by simply individually changing the state of each Tag previously known to be in the field, and using the presence or absence of a reply to update their inventory before executing a random inventory for new Tags.

For example, a SetState command includes spinup bits (4-bit, “0000”) Manchester high violation (1-bit), command bits (8-bit, “11000001”), Session number [S] (2-bit), State flag (1-bit), Tag manipulation flags (2-bit, “00” for setting state if match and setting opposite state if not match, “10” for setting state if match mask but do nothing if not match, “11” for setting state if not match mask but do nothing if matching), Pointer (8-bit), Length (8-bit, Length of mask bits), Mask bits (variable size), and CRC8 (8-bit, calculated from the first command bit through the last mask bit).

In response to a SetState command, a Tag in the starting state of “Awaiting first query” remains in the state of “Awaiting first query”. A Tag in the starting state of “Ready”, “Selected”, “Waiting for ACK”, or “Acknowledged” enters into the state of “Ready”. In response to the SetState ACK command, a Tag sets the session number to [S] and the AB state of the session to “A” or “B” depending on the mask. When the mask matches, the Tag sends a positive reply; otherwise, the Tag does not reply. If state flag is “A” and state manipulation flags are “00”, the AB state of the session is set to “A” if match and to “B” if not match. If state flag is “A” and state manipulation flags are “01”), do nothing. If state flag is “A” and state manipulation flags are “10”, the AB state of the session is set to “A” if match and do nothing if mask doesn't match. If state flag is “A” and state manipulation flags are “11”, the AB state of the session is set to “A” if not match and do nothing if mask doesn't match. If state flag is “B” and state manipulation flags are “00”, the AB state of the session is set to “B” if match and to “A” if not match. If state flag is “B” and state manipulation flags are “01”), do nothing. If state flag is “B” and state manipulation flags are “10”, the AB state of the session is set to “B” if match and do nothing if mask doesn't match. If state flag is “B” and state manipulation flags are “11”, the AB state of the session is set to “B” if not match and do nothing if mask doesn't match.

Tags matching their data to the mask sent by the Reader reply in the currently set backscatter mode, and at the currently set backscatter rate. (Both are set by a QueryA or QueryB command). They send a one, followed by a crowbar off for one bit period, 16 bits of the data from the table below, followed by another crowbar off period, and a trailing 1.

For example, a reply to a SetState command includes TAGSPINUP (1-bit, “1”), High violation (Crowbar off for FM0 or F2F bit period), Tag confirm (16-bit, “011010101010101”), High violation (Crowbar off for FM0 or F2F bit period), and TAGTRAILER (1-bit, “1”).

A SELECT command is an addressing command. The SELECTED state is held in volatile memory and is cleared at power on reset, and also cleared by the use of any Query command. Programming and class II or higher commands are divided into addressing (SELECT) and data exchange parts to allow the Tag communications hardware and registers used for addressing to be reused for reading and writing. The Tag is in the “Selected” state to execute KILL, ProgramID, VerifyID, LockID, Read and Write. (Select is only used to Kill, Program and LockID in class I Tags).

For example, a SELECT command includes spinup bits (4 bit, “0000”), Manchester high violation (1-bit), command bits (8-bit, “111000010”″, Session number (2-bit), and CRC8 (Calculated from the first command bit through the session number).

Tag addressing proceeds as follows.

-   -   1) Pick an open session.     -   2) Issue a mask for that session, specific enough to probably         get only the Tag you are looking for.     -   3) Search for the Tag using Query-ACK until you find the Tag you         are looking for (recognizing it by its complete ePC and CRC).     -   4) Issue the SELECT command.

In response to a SELECT command, a Tag in the starting state of “Awaiting first query” does not reply and remains in the state of “Awaiting first query”. A Tag in the starting state of “Ready”, “Selected”, or “Waiting for ACK” does not reply and enters into the state of “Ready”. A Tag in the starting state of “Acknowledged” provides a positive reply if the power is high enough for writing and a negative reply if the power is not high enough for writing, and enters into the state of “Selected”.

Tags selected by the select command reply in the currently set backscatter mode, and at the currently set backscatter rate. (Both are set by a QueryA or QueryB command). They send a one, followed by a crowbar off for one bit period, 16 bits from the table below, followed by another crowbar off period, and a trailing 1.

For example, a reply to a SELECT command includes TAGSPINUP (1-bit, “1”), High violation (Crowbar off for FM0 or F2F bit period), Tag confirm (16-bit, “0000 0000 0000 0000 if power is not high enough to write, “0101 0101 0101 0101 if power is high enough to write), High violation (Crowbar off for FM0 or F2F bit period) and TAGTRAILER (1-bit, “1”).

A KILL command is addressed by the SELECTED address mode. Tags matching the kill code sent by the Reader in the [VALUE] field are deactivated and no longer respond to Reader queries. Any bits beyond the length of the kill code supported by the Tag are ignored except for the CRC calculation, and if all the bits in the Tag match the kill code, the kill command executes the appropriate kill. It is anticipated that the KILL command requires higher field strengths from the Reader, and therefore be a short-range operation. The Reader transmits “1”s for 100 milliseconds, then 100 milliseconds of “0”s, followed by 15 “1”s and then another 100 milliseconds of “0”s after the kill code for the Tag to complete the command.

For example, a KILL command includes spinup bits (4 bit “0000”), Manchester high violation (1-bit), command bits (8-bit, “11000011”), Kill type (2-bit, “00” for complete kill (erase all data and permanently deactivate, “Munchkin dead”), “01” for Preserve recycle (erase all but recycling field), “10” for cloak (set to unresponsive, but not erased)), Pointer (8-bit), Length (8-bit, Length of mask bits), Kill code (variable side), and CRC8 (8-bit, calculated over the bits from the first command bit through the full kill code, including any ignored bits of the kill code).

A Tag is first in the selected state for the kill command to be executed. Tags ignore kill code data beyond the length that it can handle. If the kill code matches the bits it does have, it executes kill. (Longer kill code Tags are more secure, shorter kill code Tags may be cheaper, all Tags compatible).

In response to a KILL command, a Tag in the starting state of “Selected” sets kill bits to DEAD without a reply and enters in the state of “DEAD”, if kill code is matched and kill is successful. A Tag in the starting state of “Selected” sends a negative response and remains in the state of “Selected”, if kill code is matched but kill is not successful. A Tag in the starting state of “Awaiting first query” does not reply and remains in the state of “Awaiting first query”. A Tag in the starting state of “Ready”, “Waiting for ACK”, or “Acknowledged” does not reply and enters into the state of “Ready”.

Only Tags which unsuccessfully attempt to execute a KILL command reply in the currently set backscatter mode, and at the currently set backscatter rate. (Both are set by a QueryA or QueryB command). They send a one, followed by a crowbar off for one bit period, 16 bits from the table below, followed by another crowbar off period, and a trailing 1.

For example, a reply to a KILL command includes TAGSPINUP (1-bit, “1”), High violation (Crowbar off for FM0 or F2F bit period), Tag confirm (16-bit, “0000 0000 0000 0000 for Kill command failed), High violation (Crowbar off for FM0 or F2F bit period), and TAGTRAILER (1-bit, “1”).

A Tag is SELECTED to respond to the ScrollMFG command. For example, a ScrollMFG command includes spinup bits (4-bit “0000”), Manchester high violation (1-bit), command bits (8-bit, “1000100”), and CRC8 (8-bit, calculated over all the command bits)

In response to a ScrollMFG command, a Tag in the starting state of “Selected” sends a reply and remains in the state of “Selected”. A Tag in the starting state of “Awaiting first query” does not reply and remains in the state of “Awaiting first query”. A Tag in the starting state of “Ready”, “Waiting for ACK”, or “Acknowledged” does not reply and enters into the state of “Ready”.

Selected Tags reply to the SCROLLMFG command by sending back the preamble, and the following data, which is never allowed to be programmable. The SCROLLMFG reply can optionally end at any field after the MANUFACTURER field.

For example, a reply to a ScrollMFG command includes Preamble (4-bit, “0000”), High Manchester violation, MANUFACTURER (16-bit, assigned by “authorizing agency), MASK SET/PRODUCT CODE (16-bit, manufacturer defined), DIE NUMBER (16-bit, manufacturer defined), CAPABILITY CODE (16-bit, assigned by “authorizing agency), MEMORY SIZE (16, capability code dependent meaning), and CRC (16-bit, calculated over all the bits from the manufacturer to the last field transmitted).

Programming Commands use the same command structure and field definitions as the Basic Commands, but are typically issued only by a Tag programming device, or Programmer. A Tag Programmer may be similar to a Reader, with the exception that it can execute Programming Commands in addition to Basic Commands, in accordance with methods approved by Tag (and IC) manufacturers.

Programming Commands enable programming of the contents of the Tag memory, and verification of the contents of the Tag memory prior to locking the contents.

Manufacturers may define additional optional commands which are specifically used for manufacturing test only. These commands are required to have command codes in the range D7h to Dfh.

All Programming Commands are disabled once the Tag manufacturer has locked the Tag data contents. The specific timings for programming a Tag are memory technology dependent.

Tag programming is accomplished 16 bits at a time. Programming is typically allowed if the Tag has not been previously locked. If a Tag is not known to be cleared or of a type not needing an erase cycle before programming, EraseID is used before ProgramID.

The data is sent to the Tag using the ProgramID Command, where the [PTR] field is the memory row address to be programmed and the [VAL] field contains the 16 bits of data to be programmed into the selected memory row address.

Upon receipt of a valid ProgramID Command, the Tag executes the appropriate internal timing sequences required to program memory.

For example, a ProgramID Command includes spinup bits (4-bit “0000”), Manchester high violation (1-bit), command bits (8-bit, “11000101”″), pointer (8-bit), Data Area (2-bit, “00” for CRC & ePC, “01” for user data (none for class I), “10” for kill code), Length (8-bit, Length of data), ID to program (variable size), and CRC8 (8-bit, calculated over all fields from the first command bit to the end of the ID).

In response to a ProgramID command, a Tag in the starting state of “Selected” writes data if it is not locked and remains in the state of “Selected”. A Tag in the starting state of “Selected” sends a positive reply if successful and a negative reply if unsuccessful. A Tag in the starting state of “Awaiting first query” does not reply and remains in the state of “Awaiting first query”. A Tag in the starting state of “Ready”, “Waiting for ACK”, or “Acknowledged” does not reply and enters into the state of “Ready”.

Tags executing the ProgramID command reply in the currently set backscatter mode, and at the currently set backscatter rate. (Both are set by a QueryA or QueryB command). They send a one, followed by a crowbar off for one bit period, 16 bits from the table below, followed by another crowbar off period, and a trailing 1.

For example, a reply to a ProgramID command includes TAGSPINUP (1-bit, “1”), High violation (Crowbar off for FM0 or F2F bit period), Tag confirm (1 6-bit, “0000 0000 0000 0000 for failed to write, “0101 0101 0101 0101 for successful write), High violation (Crowbar off for FM0 or F2F bit period), and TAGTRAILER (1-bit, “1”).

Tag Erasing is accomplished 16 bits at a time. Erasing the ID is only allowed if the Tag has not been previously locked. Upon receipt of a valid EraseID Command, the Tag executes the appropriate internal timing sequences required to program memory.

For example, an EraseID Command includes spinup bits (4 bit “0000”), Manchester high violation (1-bit), command bits (8-bit, “110001 11”), and CRC8 (8-bit, calculated over all fields from the first command bit to the end of the ID).

In response to an EraseID command, a Tag in the starting state of “Selected” tries to erase ePC and CRC if it is not locked and remains in the state of “Selected”. A Tag in the starting state of“Selected” sends a positive reply if successful and a negative reply if unsuccessful. A Tag in the starting state of “Awaiting first query” does not reply and remains in the state of “Awaiting first query”. A Tag in the starting state of “Ready”, “Waifing for ACK”, or “Acknowledged” does not reply and enters into the state of “Ready”.

Tags executing the EraseID command reply in the currently set backscatter mode, and at the currently set backscatter rate. (Both are set by a QueryA or QueryB command). They send a one, followed by a crowbar off for one bit period, 16 bits from the table below, followed by another crowbar off period, and a trailing 1.

For example, a reply to an EraseID command includes TAGSPINUP (1-bit, “I), High violation (Crowbar off for FM0 or F2F bit period), Tag confirn (16-bit, “0000 0000 0000 0000 for failed to erase, “0101 0101 0101 0101 for successful erase), High violation (Crowbar off for FM0 or F2F bit period), and TAGTRAILER (1-bit, “1”).

A VerifyID command scrolls out the entire contents of memory, including protected fields. The Tag does not respond to VerifyID after it is locked. The Tag is SELECTED before VerifyID can be executed.

For example, a VerifyID command includes spinup bits (4 bit “0000”), Manchester high violation (I-bit), command bits (8-bit, “11001000”), and CRC8 (8-bit, calculated over all the command bits).

In response to an EraseID command, a Tag in the starting state of “Selected” replies and remains in the state of “Selected”, if it is not locked. A Tag in the starting state of “Selected” does not reply and enters into the state of “Ready”, if it is not locked. A Tag in the starting state of “Awaiting first query” does not reply and remains in the state of “Awaiting first query”. A Tag in the starting state of “Ready”, “Waiting for ACK”, or “Acknowledged” does not reply and enters into the state of “Ready”.

Tags matching their 16-bit handshake to that sent by the Reader reply in the currently set backscatter mode, and at the currently set backscatter rate. (Both are set by a QueryA or QueryB command). They respond by sending four (4) one (1) bits followed by a high violation for one R-T bit time, and all the identification data in the Tag starting at bit 0. Data sent by the Reader to the Tag may be of variaole length. The data is followed by a crowbar off (high) violation, and 4 trailing ones (1)s.

For example, a reply to a VerifyID command includes TAGSPINUP (4-bit, “1111”), High violation (Crowbar off for FM0 or F2F bit period), Tag data (variable size, all Tag data contents, including protected fields), High violation, and TAGTRAILER (4-bit, “1111”).

A Locki) command is used to lock the identification (CRC and ePC) portions of a Tag's memory before it leaves a controlled supply channel. The Tag is SELECTED before LockID can be executed.

For example, a LockID command includes spinup bits (4-bit “0000”), Manchester high violation (1-bit), command bits (8-bit, “11001001”), and CRC8 (8-bit, calculated over all the command bits).

In response to a LockID command, a Tag in the starting state of “Selected” tries to lock ePC and CRC, if it is not locked. A Tag in the starting state of “Selected” remains in the state of “Selected” and provides a positive response if successful and a negative response if unsuccessful. A Tag in the starting state of “Awaiting first query” does not reply and remains in the state of “Awaiting first query”. A Tag in the starting state of “Ready”, “Waiting for ACK”, or “Acknowledged” does not reply and enters into the state of “Ready”.

Tags selected by the select command execute the LockID command and then reply in the currently set backscatter mode, and at the currently set backscatter rate. (Both are set by a QueryA or QueryB command). They send a one, followed by a crowbar off for one bit period, 16 bits from the table below, followed by another crowbar off period, and a trailing 1.

For example, a reply to a LocklD command includes TAGSPJNUP (1-bit, “1”), High violation (Crowbar off for FM0 or F2F bit period), Tag confirm (16-bit, “0000 0000 0000 0000 for fail to LockID, “0101 0101 0101 0101 for Command successfiul), High violation (Crowbar off for FM0 or F2F bit period), and TAGTRAILER (1-bit, “1”).

Class II and higher Tags use the same singulation method and identification method as the class I Tags. In addition, they may have additional read/write memory, security features, sensors, etc. Class II Tags may also have a battery to realize sensor logging, for example.

Class III and above Tags are defined to have battery-assisted communications. Class 11 and above Tags respond to class I commands in a low power passive mode, or at longer ranges they use a key transmission to go to the battery assist communications mode. The wakeup key for class three and above Tags is described in a separate section. This allow them to avoid using their battery assist except for long range exchanges which are specifically directed at them, reducing the use of battery power. F2F communications may be required for communication at long range.

Communications with class II and above Tags are standardized into a handle based I/O data link. Handles are issued by “authorizing agency for a specific purpose, and together with the SCROLLMFG information, can be used to deduce and exercise the capabilities of a Tag. A few example capability codes and corresponding handles include: 00XX XXXX XXXX XXXX for Handles and capabilities by look up table; 0 XX XXXX for Handles and capabilities by subfields; 01XX XXXX XXXX 0000 for no memory; OIXX XXXX XXXX 0001 for bit wide memory, 0-7FFFFF memory address in bits, Read LEN bits starting at given address, Write LEN bits at given address; 01XX XXXX XXXX 0010 for Byte wide memory, 0-7FFFFF Memory addresses in bytes, Read LEN Bits starting at given address, Write LEN bits at given address; 0XX XXXX XXX XXXX for Scratchpad type Memory, such as 1)1-7FFFFF Memory addresses, Read LEN Bits starting at given address, Write address and LEN data bits to scratch pad, or 2) FFFFFF, Verify scratchpad data and address, or 3) FFFFFE, Write scratch pad to memory; 01XX XXXX OOOX XXXX for no security; 01XX XXXX 001×XXXX for Key exchange security, such as 1) FFFFFD, Write security token, LEN bits long, or 2) FFFFFC, Read security token; 01XX XXOO XXXX XXXX for No temperatures; OIXX XX01 XXXX XXXX for Temperature interval recorder, such as 1) FFFFFFB Interval, Set interval in seconds, Read current interval, or 2) FFFFFFA, Set number of temperatures to read at a time, or 3) FFFE 0000 0000-FFFE FmFb FmFF, Read temps, Handle- FFFEOOOOOOO 0 intervals into past.

A READ command is the basic command for fetching data from the class II or higher Tag. It is addressed only by the SELECTED addressing mode, and uses a 24 bit PTR field which is a read address or handle, and a LEN field which is the number of bits to read, or is used as a second parameter whose meanings are to be determined by the handle.

For example, a READ command includes spinup bits (4-bit “0000,), Manchester hign violation (i-bit), command bits (8-bit, “11010000”), Read handle (24-bit, Meaning defined by capability code), Data Area (2-bit, “00” for CRC & ePC, “Ol” for user data (none for class I), “1 0” for kill code), Length (8-bit, length of data to read), and CRC8 (8-bit, calculated over all fields from the first command bit to the end of length).

The data returned by the Tag is determined by the capacity code and the handle used. It possibly contains a CRC.

For example, a reply to a read command includes TAGSPINUP (1-bit, “1”), High violation (Crowbar off for FM0 or F2F bit period), Data (variable size data), High violation (Crowbar off for FM0 or F2F bit period), and TAGTRAILER (I-bit, “1”).

A WRITE command is the basic command for writing data to the class II or higher Tag. It is addressed only by the SELECTED addressing mode, and uses a 24-bit PTR field which is a read address or handle, a 16 bit LEN, and a variable length data field whose length is determined by the [LEN] parameter.

For example, a WRITE command includes spinup bits (4-bit “0000,), Manchester high violation (1-bit), command bits (8-bit, “1010001”), Write handle (24-bit, meaning defined by capability codes), Length (8-bit, length of data (granularity defined by handle & capability codes)), Data to write (variable size), and CRC8 (8-bit).

Tags selected by the select command execute the WRITE command, and reply in the currently set backscatter mode, and at the currently set backscatter rate. (Both are set by a QueryA or QueryB command). They send a one, followed by a crowbar off for one bit period, 16 bits from the table below, followed by another crowbar off period, and a trailing 1.

For example, a reply to a WRITE command includes TAGSPINUP (1-bit, “1”), High violation (Crowbar off for FM0 or F2F bit period), Tag confirm (16-bit, “0000 0000 0000 0000 for write failed, “010 0101 0101 0101 for write successful), High violation (Crowbar off for FM0 or F2F bit period), TAGTRAILER (1-bit, “1”).

Thus, advantages of one implementation of the present invention includes: four independent sessions, one session (zero) implemented with a persistent node; Match/not match masking independent in each session; Build up masking (Tag set unions, etc); Unmasked latecomers included/excluded in masked session both possible (by using a non-persistent state B, or state A); Short coherence time, the time needed to count a particular Tag from the time it powered up; No choke point (N log N); Efficient use of Reader to Tag channel, 49.25 Reader to Tag bits per Tag inventoried (typical); 16+16+16+Prefix+CRC+ePC Tag to Reader bits per Tag inventoried (average), including 16 bits for no Tag response, 16 bits for legible response to query and 16 bits for collision in bin; Tag bits concealed from Reader emissions, for commercial data protection; and, Capable of Simultaneous multi-Reader operation (when set to complete atomicity and Query-ACK-select commands grouped).

One embodiment of the present invention includes the use of the two-state symmetry of the protocol, which has advanTages over a ready-quiet alternative. The symmetric version effectively has less state dependence by symmetrizing the quiet-ready states into two symmetrical halves, the State A and State B of the protocol.

The symmetry described in this description substantially increases the performance over ready-quiet protocols in cases where Tags have been inventoried and put into the quiet state, and it is desired to inventory them again, either from a different Reader station, or as part of a continuous inventory to monitor Tags being removed on a timely basis.

In the case of a ready-quiet protocol, the Tags, once placed in the quiet state, are touched by a talk command before they can participate in an inventory. Several talk commands can be issued before an inventory, but there is no guarantee that multi-path are favorable, the frequency right, or even that the Tag is physically close enough to the Reader at that point in time. By eliminating the need to see a talk command, a Tag can be counted during a single “lucky” time or position, extending the effective reliable range of the protocol.

The use of a time out for a persistent quiet is a potential simpler alternative, but the manufacture of a Tag which has a tightly controlled persistence time is difficult. Also, for example, 10 seconds might be too short a time to inventory a large number of Tags, and may yet 30 seconds might be long enough to interfere with multiple Readers tracking an item on a trajectory or catching a shoplifter in the act of destroying a Tag or putting the item into a shielded bag and walking away with an item.

One recommended implementation of the Tag is to supply a persistent node which maintains its state for at least twenty seconds even in the absence of power. Assuming that the persistent node decays to the zero (0) state, [0] encodes state A, and [1] encodes state B. State B expires with time into the state A. There is no upper bound on the time that state B persists, but it is not permitted to be in states where it is randomly powering up into state A or state B. The suggested implementation is to refresh the persistent node at some time during every command, weather or not this Tag is addressed.

Readers would start by doing an inventory as described in the previous section, using QueryA or QueryRep commands, and ACKs. After no further Tags are responding, the Reader would continue to do high level Query commands to explore any uncounted Tags. Note that a Tag in state A would be counted even if it were only powered for a brief time, just long enough to see a command to sync its clock, a QueryA, an ACK, and one subsequent command. At this point all Tags which have been inventoried are in state B. After a predetermined amount of time, a new inventory would be done, in the same way, but using QueryB. Note that there is no need to do any separate talk or wake commands, as all Tags that are powered at that moment have been placed into state B already. After this inventory, all inventoried Tags are in the state A, and the Reader can continue to do high level QueryA commands for a while. Then an A inventory could start again, with no need to issue talk commands, and there is no possibility that a talk command is missed.

Any Tag that is continuously in the field is counted in every inventory, both the A and B flavors. Any Tag that enters the field are counted, in worst case, in the second inventory after it enters the field, the same worst case time as if a ready-quiet protocol was used, even if a talk command at the beginning of each inventory was guaranteed to be received in the quiet-talk type protocol.

A persistent quiet capability makes a very important contribution to the capabilities of an RFID system, specifically consistency in counting Tags near the edge of range when Tags are moving through the Reader field. For Tags at the edge of range, as the frequencies are changed and multi-path interference changes as the Tag or other objects are moved in the Reader field, the power available fluctuates and may only be sufficient to power the Tag for brief periods of time. Persistent sleep allows the majority of Tags to be counted quickly, and then for repeated Q=O Queries to be generated over and over again, seeking out Tags which are only intermittently powered. The symmetrical commands extend this comprehensive counting capability to Tags have just been inventoried and put into the quiet state and would therefore potentially missed if they did not receive a wakeup command. It is also useful as part of a continuous inventory process.

The main advantage of this approach is that it prevents the Tags from ever entering a state where they are hard to count, which they are in the quiet state of a ready-quiet protocol. At first glance, it may seem that the quiet state is not hard to get out of, because a talk command can be used. However, if a Tag is unknown to the Reader, only a high level talk command potentially wakes it up, and a high level talk command would wake up all the other Tags as well. It may seem that if a Tag is in a quiet state, it has been recently inventoried anyway, but if a different Reader station did that inventory, or if a continuous inventory is desired, then it needs to be inventoried again. If a Tag is in the quiet state in a ready-quiet protocol, it is touched twice, at two widely separated times and possibly two frequencies. It needs to be active once when the whole field of Tags is awakerned, and then again later when that particular Tag is to be inventoried. Needing two events to happen greatly impacts the probability of counting Tags which are on the margin and are only intermittently powered, and that is avoided by using the symmetric protocol. The time savings of not requiring additional talk commands to be issued is a small side benefit.

A population of falling edge decoder only split phase Manchester Tags can also be run by a 75% duty cycle stream. For example,

% Manchester shown as NRZ: [001761 1101011101101101011011101 1001771 50% Manchester shown at half the data rate in NRZ: 1001781 1110010011111001110011 10010011100111100I

Falling edges maintained, duty cycle increased: 1110111111111011110111101111111011111101I This runs at half the rate with the same spectra (some reduction in amplitude), provides a minimum of 75% average Tag power, and with half mum low time of the high rate split phase Manchester, and decodes using cal rising edge Manchester decoder. It requires the same filter at the Reader the 50% Manchester stream shown originally. A population of falling edge decoder only split phase Manchester Tags at high rates while reducing the maximum low time, and increasing rage power by the following method. 50% Manchester shown as NRZ: 11010111010!i1ololo1101 101101110101110101111101 Falling edges maintained, and low time reduced: 1101111101101101 1101110110111110101 1111101101 This decodes with the same falling edge Manchester decoder, and runs at te for the same spectra (with a slight change in sideband amplitude only). e maximum low time of the 50% split phase Manchester. This encoding e same filter at the Reader output as the 50% Manchester stream. It verage power at the Tags. A rising edge detector can detect the Manchester violation after the regardless of the following data. A Manchester rising edge decoder (in keeping its clock synced) usually only needs to detect whether there is a at the center of a bit, if there isn't a one. To detect a Manchester high e logic needs to check the time to the next rising edge. A time of 2 bit periods or more means there was a Manchester violation.

For example, [001901 0.1101101101111101101ollllllll

- A A A A A Manchester violation 1001921 011011011101111101011111111111

A A - A A Manchester violation (001941 0110110110111110110110111111111

A A A A A A Manchester violation

0110110110111111o01011101111111

A A A A A A Manchester violation 1001991- - A - - A - A No Manchester violation 1002001 01101101101110110101110o1111111 1002011 . . . . A A A No Manchester violation

In one embodiment of the present invention, the RF Transport Layer is designed for UHF operation, although other alternative operations can also be used. The Tag-to-Reader and Reader-to-Tag link is defined as half-duplex. Readers initiate communication with Tags by sending a command sequence. The Reader then provides a reply period for the Tag by transmitting an un-modulated carrier while listening for a reply from the Tag. The Tags are not expected to be able to detect Reader modulation while they are replying. The RF transport layer may use the following bands: 902-928 MHz and 2400.0-2483.5 MHz in North American, 869.4-869.65 MHz and 865.6-867.6 MHz in European, and UHF bands near 2450 MHz. -These UHF bands have features including: High speed (up to 160 Kbps in North America) Reader-to-Tag system data rate; 320 kbps or faster Tag-to-Reader system data rate with consistent operation with high “hit rates at ranges of 2 meters or more under typical conditions, 5 meters or more in an anechoic chamber, relatively small Tag and Reader antennas; human exposure to low-level UHF is already common and accepted worldwide (i.e. cellular telephones); and, North American bands are wide enough to permit significant frequency hopping.

Readers are in one of three available states: OFF (emitting no RF energy), CW (emitting RF energy at some power level without amplitude modulation), and Active (emitting RF energy with amplitude modulation).

FIG. 10 illustrates Reader to Tag Modulation. In the Active state, the Reader-to-Tag link makes use of split phase Manchester encoding with a minimum modulation depth of 30%. A data one (1) is encoded by a high RF period followed by a low RF period. A data zero (0) is encoded by a low RF period followed by a high RF period. Modulation shape, depth and rate of modulation are variable within the limits described below. Tags adjust their timing over a range of modulation rates to lock to Reader transmissions automatically. Falling edges are to be held to 1% To of their nominal positions, but rising edges may vary from their nominal positions, with a minimum CW off time of 3 microseconds.

The modulation parameters in general are illustrated in FIG. 11. It is understood that specific values of pulse modulation parameters can be a function of the local regulatory environment. For example, pulse modulation parameters includes:

To: Elemental Clock Cycle period. Time for a single bit sent from Reader to Tag.

T.: Rise time of modulation envelope, 10% to 90% of Modulation amplitude variation.

Tf: Fall time of modulation envelope, 90% to 10% of Modulation amplitude variation

Tim: Pulse width of modulation envelope measured at 50% of Modulation amplitude variation.

Mod: Amplitude variation of modulated carrier.

Ripple: Peak to Peak variation in modulation overshoot and undershoot at edges of intended modulation.

Dm: Peak to Peak variation in modulation depth from intended value.

TOToI: Master Clock Interval Tolerance, Basic accuracy of Reader signaling.

TCW: Minimum CW time immediately before a command.

Tcoast: Maximum Time duration between EOF and the next command, to ensure that the Tag clock is sufficiently accurate to decode the next command.

In one implementation, for Reader modulation sequences, the Reader clock is stable to within 1% over the length of a transaction. All other “Elemental Clock Cycle timings are proportional to the modulation clock frequency To.

In one implementation, modulation parameters have the following values, in which All times and frequencies scale with To, except as noted.

To: Master Clock Interval (6 us to 24 us) -TOTOI: Master Clock Interval Tolerance (+0.1% Maximum) DR: Data Rate (l/To) T1ejiw: Maximum jitter from nominal in falling edges (0.01 *To) DutyCycle: RF high period duty cycle (=50% or >50%/o) MOD: Modulation Depth (30% minimum) Tf: Max fall Time (1/4 To) T,: Max Rise Time (1/4 To) Ripple: Ripple (10% pp) TCW: Minimum CW time preceding any command, may overlap with W interval (4×To) Tcat: Time duration between EOF and the next command (5 ms max., ale with To) FIG. 12 illustrates an example of Reader to Tag modulation encoding. ementation, all transactions begin with a minimum CW period preceding nd to allow the Tag to locate the beginning of the command. All tart with 4 spin up bits to synchronize the Tag clocks. During the Data of a command, Tags maintain their clock phase by reference to the of the Reader to Tag data modulation, which are held to low time jitter. d time, To, determines the Reader-to-Tag data rate. inary data from Reader to Tag is Manchester encoded. Logical zeros s a low period nominally (1/2) of the master clock interval, To, followed RF high period. Logical ones are encoded by a high RF period 2) of the master clock interval, To followed by a nominally (1/2) To RF—-low period. The falling edges are held to a low amount ofjitter from their nominal times as defined by the Manchester coding. The fall times can vary from their nominal value to accommodate a RF duty cycle.

After the last pulse, the Tag is ready to receive the next command after the minimum CW time, and be able to decode a command received within a Tcot interval.

In order for the Tag to be able successfully decode the next command; the Reader starts the next transaction within the Tcot interval. This restriction does not apply when the carrier has been turned off for sufficient time for the Tag to loose DC power, as the Tag re-synchronizes at the next power up. If the Tag clock frequency is adjusted by more than 20% in a spin up, the Tag does not respond to that command.

The Reader to Tag data is Manchester encoded, with a data one (1) encoded by a high RF half bit followed by a low RF half bit, and a data zero (0) encoded by a low RF half bit followed by a high RF half bit. The falling edges held to low jitter. The rising edges are allowed to deviate from their nominal positions to accommodate different duty cycles.

FIG. 13 illustrates data modulation timing for data “0”, “1” and Manchester high volation. Data Modulation Timing, Tdato, for Reader-to-Tag clocking when data =“0” is encoded by an RF low period followed by a high period. Data Modulation Timing, Tdatal, for Reader-to-Tag clocking when data =“I” is encoded by an RF high period followed by an RF low period. Data Modulation Timing, Tdml, for Reader-to-Tag clocking for the Manchester high violation which occurs at the end of the spinup bits.

The Reader may optionally shorten the time between commands below that required for a response. The Reader may listen for a Tag reply during the reply period, and if a reply has NOT been started by a Tag, prior to expiration of a time (TTacmID,1+2*TO), shorten the duration for that reply interval.

Tags reply to Readers by means of modulating their backscatter between two states. These two states may modulate the backscatter in phase or amplitude, or both.

One of these is assumed, but not required, to impair the energy gathering ability of the Tag, and is referred to in this document as the crowbar on state. It is assumed that the backscatter state of the Tag is “crowbar off until the start of backscatter modulation. As many Tags need to return to the “crowbar off state at the end of backscatter modulation, all Tags are required to return their backscatter state to the state that they were in prior to starting a backscatter reply. This transition occurs after the end of the last backscatter transition tmmsmitted (to the crowbar on state), at a time equal to that of the smallest feature size in the backscatter modulation mode.

The Tag-to-Reader modulation is selected by the [MODULATION], and is either FMO in the case of [MODULATION]=O, or F2F, in the case of [MODULATION]=1, or an as yet undefined high frequency mode for [MODULATION]=3. All Tags are required to implement all three forms of encoding. Readers may implement one or more decoders. The primitive for F2F method is the same, with the same timing, as FMO, but two FMO primitives are used to encode each bit of F2F.

Note that the F2F encoder can be implemented using a FMO encoder preceded by a bit inverter and encoding twice for each bit in succession.

FMO: Tags reply to Reader commands with backscatter modulation. There is a change of backscatter state between each bit period, and a 0 bit has an additional change of backscatter state at the center of the bit time. The nominal data rate for Tag to Reader is four times the Reader to Tag Rate, but may vary up to ±10% over an 80-bit response window due to oscillator drift in the Tag. It is anticipated that FMO are used in cases where the backscatter noise environment can be controlled, i.e. shielding around fluorescent lamps, etc.

F2F: Tags reply to Reader commands with backscatter modulation that follows a four (4)-interval bit cell encoding scheme. Two (2) transitions are observed for a zero (0) and four (4) transitions are observed for a one (1) during a Bit Cell. The nominal data rate for Tag to Reader is twice the Reader to Tag Rate, but may vary up to I10% over an 80 bit response window due to oscillator drift in the Tag.

Some examples of the Tag to Reader modulation parameters are listed below.

To: Reader to Tag Master Clock Interval;

TTagbitc,ll: Tag to Reader bit cell interval (To/4 for FMO, To/2 for F2F);

Tag Data Rate: Tag to Reader Nominal Data Rate (⁴/To for FMO, ²/To for F2F);

TTg.mID~I: Reply delay from end of command to start of Tag ScrollID Reply (2×To);

TTagDI: Reply delay from end of command to start of Tag ID Reply (2×To);

-   -   TTagreplyNom: Tag to Reader reply duration for 8+16+96 bit         ScrolID reply (TTagbitcelI x 120 bits);

ATTagbitcel: Tag to Reader bit cell interval variation at last bit of 120 bit Scrolli) reply (10% max); [00250J Tcoast: Time duration between EOF and the next command (5 ms max.).

The delay from the end of data to the start of a reply to a ScrollID or VerifyID Command, TTagWIIDcI, is illustrated in FIG. 14. The duration of a ScrollID Reply, TTagMplyNom is illustrated in FIG. 14. The variation in the bit cell duration ATTagbitc,ii is illustrated in FIG. 15.

Tag-to-Reader bit cell encoding is illustrated in FIG. 16. FMO: The state of Tag backscatter is changed at the edge of each bit, and an additional change of state occurs in the middle of bit intervals which correspond to a “0” data bit. The crowbar starts in the off state, and makes its first transition to the on state at the beginning of the first data bit. A final bit interval transition is inserted at the end if it is needed to leave the modulation in the high state (not in the crowbar state), at the later edge of potentially one extra bit time.

F2F: The Tag backscatter is modulated by a selection of one of two symbols per bit cell.

Under this encoding scheme there are always transitions in the middle of a bit and unlike Manchester encoding, the sense of zeros and ones are maintained when the code is inverted. FIG. 17 illustrates this inversion.

It will be apparent from this description that aspects of the present invention may be embodied, at least in part, in software. That is, the techniques may be carried out in a computer system or other data processing system in response to its processor, such as a microprocessor, executing sequences of instructions contained in a memory, such as memory 111. In various embodiments, hardwired circuitry may be used in combination with software instructions to implement the present invention. Thus, the techniques are not limited to any specific combination of hardware circuitry and software nor to any particular source for the instructions executed by the data processing system. In addition, throughout this description, various functions and operations are described as being performed by or caused by software code to simplify description. However, those skilled in the art will recognize what is meant by such expressions is that the functions result from execution of the code by a processor, such as the microprocessor 113.

A machine readable medium can be used to store software and data which-when executed by a data processing system causes the system to perform various methods of the present invention. This executable software and data may be stored in various places including for example memory 113 or 319. Portions of this software and/or data may be stored in any one of these storage devices.

Thus, a machine readable medium includes any mechanism that provides (i.e., stores and/or transmits) information in a form accessible by a machine (e.g., a computer, network device, personal digital assistant, manufacturing tool, any device with a set of one or more processors, etc.). For example, a machine readable medium includes recordable/non-recordable media (e.g., read only memory (ROM); random access memory (RAM); magnetic disk storage media; optical storage media; flash memory devices; etc.), as well as electrical, optical, acoustical or other forms of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.); etc.

Some of the abbreviations used in this description are listed below.

AM: Amplitude Modulation;

-   -   API: Application Programming Interface;     -   ASK: Amplitude Shift Keying;     -   bps: Bits per second;     -   CCAG: Cheap Chip Action Group;     -   CRC: Cyclic Redundancy Check;     -   CW: Continuous Wave;     -   DLL: Data Link Layer (OSI Model);     -   EME: Electromagnetic Exposure;     -   ESD: Electrostatic Discharge;

FCC: Federal Communications Commission;

-   -   FM: Frequency Modulation;     -   FSK: Frequency Shift Keying; 1002721 kbps: kilobits per second;     -   kHz: kilohertz (103 Hertz);

FM: Frequency Modulation;

-   -   HBM: Human Body Model;     -   LSB: Least Significant Bit; ms: milliseconds (10-3 seconds);         MHz: megahertz (106 Hertz); MSB: Most Significant Bit; ns:         nanoseconds (10-9 seconds); ppm: Parts per million (10I); PM:         Phase Modulation; PSK: Phase Shift Keying; RAM: Random Access         Memory; RF: Radio Frequency; RFID: Radio Frequency         Identification; RTF: Reader Talks First; us: microseconds (10-6         seconds); VLC: Very Low Cost; and V/m: Volts per Meter. Further         descriptions about detailed examples of embodiments of the         ention can be found in Appendices A and B. In the foregoing         specification, the invention has been described with specific         exemplary embodiments thereof. It will be evident that various         ns may be made thereto without departing from the broader spirit         and invention as set forth in the following claims. The         specification and e, accordingly, to be regarded in an         illustrative sense rather than a nse.—CLAIMS 

1. A method for identifying tags, the method comprising: issuing a first set of commands to identify a first plurality of tags in a first state, wherein, as a result of identifying the first plurality of tags in the first state, the first plurality of tags which are identified in the first state are placed in a second state; and issuing a second set of commands to identify a second plurality tags in a second state, wherein, as a result of identifying the second plurality of tags in the first state, the second plurality of tags which are identified in the second state are placed in the first state.
 2. A method as in claim 1 wherein issuing the first set of commands and issuing the second set of commands is performed without requiring that an interrogator know that it has placed certain tags in the first state or the second state.
 3. A method for identifying tags, the method comprising: issuing a set of commands capable of being received by a group of tags, the set of commands including at least a first command which causes an identified tag which is in a first state to be placed into a second state and a second command which causes an identified tag which is in the second state to be placed into the first state; receiving identification information from at least one tag in response to the set of commands wherein the issuing is performed without attempting to place the group of tags in a state upon initially issuing the set of commands.
 4. A method as in claim 3 wherein the identification information comprises an electronic product code.
 5. A method as in claim 3 wherein the identification information comprises a checksum.
 6. A method as in claim 3 wherein the identification information comprises a pseudorandom number.
 7. A method of identifying tags, the method comprising: issuing a first set of commands of a first type to identify tags in a first group of tags which are in a first state, wherein a first command in the first set of commands of the first type causes addressed tags to be placed in a second group of tags, which are in a second state, which are addressable by a second set of commands of a second type, wherein the first group of tags do not respond to the second set of commands of the second type; and issuing the second set of commands of the second type to identi,f tagsin the second group of tags in the second state, wherein a second command in the second set of commands of the second type causes addressed tags to be placed in the first group of tags which respond to the first set of commands of the first type and wherein the second group of tags do not respond to the first set of commands of the first type.
 8. A method as in claim 7 wherein a third group of tags in a third state are responsive to either the first set of commands of the first type or the second set of commands of the second type, the method further comprising: issuing the first set of commands of the first type to identify tags in the third group of tags which are in the third state, wherein the first command in the first set of commands of the first type causes addressed tags to be placed in the second group of tags in the second state; and issuing the second set of commands of the second type to identify tags in the third group of tags which are in the third state, wherein the second command in the second set of commands of the second type causes addressed tags to be placed in the first group of tags in the first state.
 9. A method as in claim 7, further comprising: issuing a third command in the first set of commands of the first type to address tags in the first group of tags in the first state, wherein the third command in the first set of commands of the first type causes addressed tags to be placed in the third group of tags in the third state; and issuing a fourth command in the second set of commands of the second type to address tags in the second group of tags in the second state, wherein the fourth command in the second set of commands of the second type causes addressed tags to be placed in the third group of tags in the third state.
 10. A method as in claim 9 wherein the third command in the first set of commands causes only a single addressed tag in the first group of tags to be placed in the third group of tags and the fourth command in the second set of commands causes only a single addressed tag in the second group of tags to be placed in the third group of tags.
 11. A method as in claim 7 wherein the first command in the first set of commands causes only a single addressed tag in the first group of tags to be placed in the second group of tags and the second command in the second set of commands causes only a single addressed tag in the second group of tags to be placed in the first group of tags.
 12. A method for identifying items, comprising: sending a first interrogation signal of a first type to a plurality of items to be identified, the plurality of items in a first state; classifying those of the plurality of items to be identified which respond to the first interrogation signal of the first type; sending one or more subsequent interrogation signals of the first type to at least some of those of the plurality of items to be identified which responded to the first interrogation signal of the first type; sub-classifying those of the plurality of items to be identified which respond to the one or more subsequent interrogation signals of the first type until each sub-class has a single member, identifying each of those of the plurality of items to be identified which is the single member of the each one of the sub-classes; changing the state of the identified items to a second state, the second state preventing the identified items from responding to interrogation signals of the first type and enabling the identified items to respond to interrogation signals of a second type; interrogating, classifying and identifying those of the plurality of items to be identified which remain in the first state, with interrogation signals of the first type; and returning the plurality of items in the second state to the first state.
 13. A method performed in a tag, the method comprising: receiving at a tag in a first state, a first set of commands to identify the tag in the first state, wherein, as a result of being identified, the tag in the first state is placed in a second state receiving at the tag in the second state, a second set of commands to identify the tag in the second state, wherein, as a result of being identified, the tag in the second state is placed in the first state.
 14. A method performed in a tag, the method comprising: receiving at a tag in a first state a first set of commands of a first type to identify the tag in a first group of tags in the first state, wherein a first command in the first set of commands of the first type causes the tag in the first state to be placed in a second state in a second group of tags in the second state, which are addressable by a second set of commands of a second type, wherein the second group of tags in the second state do not respond to the first set of commands of the first type; and receiving at a tag in the second state the second set of commands of the second type to identify the tag in the second group of tags in the second state, wherein a second command in the second set of commands of the second type causes the tag in the second state to be placed in the first state in the first group of tags in the first state, which are addressable by the first set of commands of the first type, wherein the first group of tags in the first state do not respond to the second set of commands of the second type.
 15. The method of claim 14 further comprising: receiving at a tag in a third state the first set of commands of the first type to identify the tag in a third group of tags in the third state, wherein the first command in the first set of commands of the first type causes the tag in the third state to be placed in the second state in the second group of tags; and receiving at the tag in the third state the second set of conunands of the second type to identify the tag in the third group of tags which are in the third state, wherein the second command in the second set of commands of the second type causes the tag to be placed in the first state in the first group of tags.
 16. The method of claim 14, further comprising: receiving at the tag in the first state a third command in the first set of commands of the first type, wherein the third command in the first set of commands of the first type causes the tag to be placed in the third state in the third group of tags; and receiving at the tag in the second state a fourth command in the second set of commands of the second type, wherein the fourth command in the second set of commands of the second type causes the tag to be placed in the third state in the third group of tags.
 17. A method performed in a tag, comprising: receiving a radio frequency signal from a reader, the signal being modulated with interrogation data representing at least a portion of an identification code, wherein the interrogation data comprises a first set of commands of a first type to identify tags in a first group of tags which are in a first state, wherein a first command in the first set of commands of the first type causes addressed tags to be placed in a second group of tags, which are in a second state, which are addressable by a second set of commands of a second type, wherein the first group of tags do not respond to the second set of commands of the second type; and wherein the interrogation data further comprises the second set of commands of the second type to identify tags in the second group of tags in the second state, wherein a second command in the second set of commands of the second type causes addressed tags to be placed in the first group of tags which respond to the first set of commands of the first type and wherein the second group of tags do not respond to the first set of commands of the first type; detecting the interrogation data representing at least a portion of an identification code; comparing the interrogation data to determine if the interrogation data matches the at least a portion of the identification code imbedded in a memory in the tag; generating a response code if the interrogation data matches the at least a portion of the identification code imbedded in the memory of the memory in the tag; modulating the response code onto a radio frequency signal; and transmitting the response code to the reader.
 18. An identification system, comprising: a plurality of identification tags in a first state, each of the plurality of identification tags comprising; a memory having an imbedded identification code; a first receiver to receive a first interrogation signal of a first type, to receive one or more subsequent interrogation signals of the first type, and to receive a state command which places the identification tag in a temporary second state wherein the identification tag is not responsive to interrogation signals of the first type and is only responsive to interrogation signals of a second type; a correlator to compare the first interrogation signal of the first type with at least a portion of the imbedded identification code, and to compare the one or more subsequent interrogation signals of the first type with one or more greater portions of the imbedded identification code; a first controller to determine when to send a first response signal if the first interrogation signal of the first type matches the at least a portion of the imbedded identification code, and to determine when to send one or more subsequent response signals if the one or more subsequent interrogation signals of the first type match the one or more greater portions of the imbedded identification code; and a first transmitter to send the first response signal and to send the one or more subsequent response signals; and a reader, comprising; a second transmitter to send the first interrogation signal of the first type to the plurality of identification tags, to send the one or more subsequent interrogation signals of the first type to the plurality of identification tags, and to send the state command to the plurality of identification tags which places at least some of the plurality of identification tags in the temporary second state wherein the at least some of the plurality of identification tags are not responsive to encoded interrogation signals of the first type and are only responsive to encoded interrogation signals of a second type; a second receiver to receive a plurality of the first response signals from the at least some of the plurality of identification tags, the plurality of the first response signals being grouped into a first plurality of time periods, the second receiver to receive a plurality of each of the one or more subsequent response signals from the at least some of the plurality of identification tags, each of the plurality of the one or more subsequent response signals being grouped into one or more subsequent pluralities of time periods; a processor to determine if more than one of the plurality of first response signals from the at least some of the plurality of identification tags have been received during a single time period of the first plurality of time periods and to determine if more than one of the plurality of one or more subsequent response signals from the at least some of the plurality of identification tags have been received during a single time period of the subsequent plurality of time periods of each of the one or more subsequent response signals from the at least some of the plurality of identification tags, wherein the second transmitter sends the one or more subsequent interrogation signals of the first type to the at least some of the plurality of identification tags whose first encoded response signal has been received during the single time period of the plurality of time periods, the subsequent interrogation signal of the first type being adapted to evoke the subsequent response signals from all of the at least some of the plurality of identification tags which are separated in time, wherein the processor uniquely identifies the all of the at least some of the plurality of identification tags whose subsequent response signals have been separated in time, and wherein the second transmitter sends the command signal which places the identification tags in the temporary second state, to the all of the at least some of the plurality of identification tags which have been uniquely identified.
 19. An apparatus to identify tags, the apparatus comprising: a processor; a transmitter coupled with the processor, the transmitter to impress identification data upon a signal that may be transmitted to a plurality of tags; an antenna coupled with the transmitter, the antenna adapted to transmit the identification data to the plurality of tags, the identification data specifying at least a portion of an identification code, wherein the transmitter is configured to issue a first set of commands of a first type to identify tags in a first group of tags which are in a first state, wherein a first command in the first set of commands of the first type causes addressed tags to be placed in a second group of tags, which are in a second state, which are addressable by a second set of commands of a second type, wherein the first group of tags do not respond to the second set of commands of the second type; and wherein the transmitter is configured to issue the second set of commands of the second type to identify tags in the second group of tags in the second state, wherein a second command in the second set of commands of the second type causes addressed tags to be placed in the first group of tags which respond to the first set of commands of the first type and wherein the second group of tags do not respond to the first set of commands of the first type; a receiver coupled with the processor, the receiver to receive at least one response from the plurality of tags, and wherein the processor is configured to identify the at least one response from the plurality of tags.
 20. A tag, comprising: an antenna to receive a radio frequency signal from a reader; the signal being modulated with interrogation data representing at least a portion of an identification code from the reader; a receiver coupled with the antenna to detect the interrogation data representing at least a portion of an identification code from the reader; a memory containing an identification code; a processor coupled with the receiver and the memory, the processor to process the interrogation data and determine if the interrogation data matches at least a portion of the identification code contained in the memory, wherein the processor generates a response code; and a transmitter coupled with the processor, the transmitter adapted to modulate the response code onto a radio frequency signal, wherein the tag transmits the response code to the reader and wherein the processor is configured to respond to a first set of commands of a first type to identify tags in a first group of tags which are in a first state, wherein a first command in the first set of commands of the first type causes addressed tags to be placed in a second group of tags, which are in a second state, which are addressable by a second set of commands of a second type, wherein the first group of tags do not respond to the second set of commands of the second type; and wherein the processor is configured to respond to the second set of commands of the second type to identify tags in the second group of tags in the second state, wherein a second command in the second set of commands of the second type causes addressed tags to be placed in the first group of tags which respond to the first set of commands of the first type and wherein the second group of tags do not respond to the first set of commands of the first type.
 21. An identification system, comprising: a plurality of identification tags in a first state, each of the plurality of identification tags comprising; means for imbedding an identification code; means for receiving a first interrogation signal of a first type; means for determining if a match exists between the first interrogation signal of the first type and at least a portion of the imbedded identification code; means for determining when to send a first response signal when the first interrogation signal of the first type matches the at least a portion of the imbedded identification code; means for sending the first response signal; means for receiving a subsequent interrogation signal of the first type to compare with a greater portion of the imbedded identification code; means for determining if a match exists between the subsequent interrogation signal of the first type and the greater portion of the imbedded identification code; means for determining when to send a subsequent response signal if the subsequent interrogation signal of the first type matches the greater portion of the imbedded identification code; means for sending the subsequent response signal; means for receiving an command which places the identification tag in a temporary second state wherein the identification tag is not responsive to interrogation signals of the first type and is only responsive to interrogation signals of a second type; and means for automatically returning the identification tag to the first state; and a reader, comprising; means for sending the first interrogation signal of the first type to the plurality of identification tags; means for receiving a plurality of the first response signals from at least some of the plurality of identification tags, the plurality of the first response signals being grouped into a plurality of time periods; means for determining if more than one of the first response signals from the at least some of the plurality of identification tags has been received during a single time period of the plurality of time periods; means for sending a subsequent interrogation signal of the first type to the at least some of the plurality of identification tags whose first response signal has been received during the single time period of the plurality of time periods, the subsequent interrogation signal of the first type being adapted to evoke subsequent responses from all of the at least some of the plurality of identification tags which are separated in time; means for uniquely identifying the all of the at least some of the plurality of identification tags whose subsequent responses have been separated in time; means for sending the command signal which places the identification tags in the temporary second state, to the all of the at least some of the plurality of identification tags which have been uniquely identified.
 22. An apparatus to identify tags, the apparatus comprising: means for issuing a first set of commands to identify tags in a first state, wherein, as a result of identifying the tags in the first state, the tags which are identified in the first state are placed in a second state; and means for issuing a second set of commands to identify tags in the second state, wherein, as a result of identifying the tags in the second state, the tags in the second state are placed in the first state.
 23. The apparatus as in claim 22 wherein the means for issuing the first set of commands and the means for issuing the second set of commands have no memory of which tags have been placed in the first state or the second state.
 24. An apparatus to identify tags, the apparatus comprising: means for issuing a set of commands capable of being received by a group of tags, the set of commands including at least a first command which causes an identified tag which is in a first state to be placed into a second state and a second command which causes an identified tag which is in the second state to be placed into the first state; means for receiving identification information from at least one tag in response to the set of commands wherein the issuing is performed without attempting to place the group of tags in a state upon initially issuing the set of commands.
 25. An apparatus to identify tags, the apparatus comprising: means for processing identification data; means for impressing identification data upon a signal that may be transmitted to a plurality of tags; means for transmitting the identification data to the plurality of tags, the identification data specifying at least a portion of an identification code, wherein the identification data comprises a first set of commands of a first type to identify tags in a first group of tags which are in a first state, wherein a first command in the first set of commands of the first type causes addressed tags to be placed in a second group of tags, which are in a second state, which are addressable by a second set of commands of a second type, wherein the first group of tags do not respond to the second set of commands of the second type; and wherein the identification data further comprises the second set of commands of the second type to identify tags in the second group of tags in the second state, wherein a second command in the second set of commands of the second type causes addressed tags to be placed in the first group of tags which respond to the first set of commands of the first type and wherein the second group of tags do not respond to the first set of commands of the first type; means for receiving at least one response from the plurality of tags.
 26. A tag, comprising: means for receiving at a tag in a first state, a first set of commands to identify the tag in the first state, wherein, as a result of being identified, the tag in the first state is placed in a second state means for receiving at the tag in the second state, a second set of commands to identify the tag in the second state, wherein, as a result of being identified, the tag in the second state is placed in the first state.
 27. A tag, comprising: means for receiving a set of commands capable of being received by a group of tags, the set of commands including at least a first command which causes an identified tag which is in a first state to be placed into a second state and a second command which causes an identified tag which is in the second state to be placed into the first state; means for sending identification information from in response to the set of commands wherein receiving the set of commands is performed without attempting to place the tag in an initial state before receiving the set of commands.
 28. A tag, comprising: means for receiving a radio frequency signal from a reader; the signal being modulated with interrogation data representing at least a portion of an identification code from the reader, wherein the interrogation data comprises a first set of commands of a first type to identify tags in a first group of tags which are in a first state, wherein a first command in the first set of commands of the first type causes addressed tags to be placed in a second group of tags, which are in a second state, which are addressable by a second set of commands of a second type, wherein the first group of tags do not respond to the second set of commands of the second type, and wherein the interrogation data further comprises the second set of commands of the second type to identify tags in the second group of tags in the second state, wherein a second command in the second set of commands of the second type causes addressed tags to be placed in the first group of tags which respond to the first set of commands of the first type and wherein the second group of tags do not respond to the first set of commands of the first type; means for detecting the interrogation data representing at least a portion of an identification code from the reader; means for imbedding an identification code; means for processing the interrogation data to determine if the interrogation data matches at least a portion of the identification code imbedded in the memory, wherein the processing generates a response code; means for modulating the response code onto a radio frequency signal which may be transmitted to the reader; means for transmitting the response code to the reader.
 29. A machine readable medium containing executable computer programming instructions, which when executed by a data processing system, cause the data processing system to perform a method for identifying tags, the method comprising: issuing a first set of commands to identify tags in a first state; issuing a second set of commands to identify tags in a second state, wherein, as a result of identifying tags in the first state, the tags which are identified in the first state are placed in the second state and wherein, as a result of identifying tags in the second state, the tags which are identified in the second state are placed in the first state.
 30. A machine readable medium as in claim 29 wherein the issuing of commands is performed without requiring that an interrogator know that it has placed certain tags in first state or the second state.
 31. A machine readable medium containing executable computer programming instructions, which when executed by a data processing system, cause the data processing system to perform a method for identifying tags, the method comprising: issuing a set of commands capable of being received by a group of tags, the set of commands including at least a first command which causes an identified tag which is in a first state to be placed into a second state and a second command which causes an identified tag which is in the second state to be placed into the first state; receiving identification information from at least one tag in response to the set of commands wherein the issuing is performed without attempting to place the group of tags in a state upon initially issuing the set of commands.
 32. A machine readable medium containing executable computer programming instructions, which when executed by a data processing system, cause the data processing system to perform a method for identifying tags, the method comprising: issuing a first set of commands of a first type to identify tags in a first group of tags which are in a first state, wherein a first command in the first set of commands of the first type causes addressed tags to be placed in a second group of tags, which are in a second state, which are addressable by a second set of commands of a second type, wherein the first group of tags do not respond to the second set of commands of the second type; issuing the second set of commands of the second type to identify tags in the second group of tags in the second state, wherein a second command in the second set of commands of the second type causes addressed tags to be placed in the first group of tags which respond to the first set of commands of the first type and wherein the second group of tags do not respond to the first set of commands of the first type.
 33. A machine readable medium containing executable computer programming instructions, which when executed by a data processing system, cause the data processing system to perform a method of identifying items, the method comprising: sending a first interrogation signal of a first type to a plurality of items to be identified, the plurality of items in a first state; classifying those of the plurality of items to be identified which respond to the first interrogation signal of the first type; sending one or more subsequent interrogation signals of the first type to at least some of those of the plurality of items to be identified which responded to the first interrogation signal of the first type; sub-classifying those of the plurality of items to be identified which respond to the one or more subsequent interrogation signals of the first type until each sub-class has a single member; identifying each of those of the plurality of items to be identified which is the single member of the each one of the sub-classes; changing the state of the identified items to a second state, the second state preventing the identified items from responding to interrogation signals of the first. type and enabling the identified items to respond to interrogation signals of a second type; interrogating, classifying and identifying those of the plurality of items to be identified which remain in the first state, with interrogation signals of the first type; and returning the plurality of items in the second state to the first state.
 34. A machine readable medium containing executable computer programming instructions, which when executed by a data processing system, cause the data processing system to perform a method of identifying a plurality of tags in a first state, the method comprising: receiving at the plurality of tags a first interrogation signal of a first type; determining if a match exists between the first interrogation signal of the first type and a first segment of the unique identification codes in at least some of the plurality of tags; sending a plurality of first response signals from the at least some of the plurality of tags when the first interrogation signal of the first type matches the first segment of the unique identification codes in the at least some of the plurality of tags, the plurality of first response signals distributed in a first plurality of time periods, each of the first plurality of time periods corresponding to a second segment of the unique identification codes in the at least some of the plurality of tags; receiving at the at least some of the plurality of tags a subsequent interrogation signal of the first type to compare with a greater segment of the unique identification codes in the at least some of the plurality of tags; determining if a match exists between the subsequent interrogation signal of the first type and the greater segment of the unique identification codes in the at least some of the plurality of tags; sending a plurality of subsequent response signals from the at least some of the plurality of tags when the subsequent interrogation signal of the first type matches the greater segment of the unique identification codes in the at least some of the plurality of tags, the plurality of subsequent response signals distributed in a plurality of subsequent time periods, the plurality of subsequent time periods corresponding to subsequent segments of the unique identification codes in the at least some of the plurality of tags; detecting when only one of the plurality of subsequent response signals is sent in one of the plurality of subsequent time periods; determining the unique identification code of the one of the plurality of tags whose subsequent response has been detected in one of the plurality of subsequent time periods; placing the one of the plurality of tags whose unique identification code has been determined into a second state wherein the tag is not responsive to interrogation signals of the first type and is only responsive to interrogation signals of a second type; and returning the one of the plurality of tags to the first state.
 35. A machine readable medium containing executable computer programming instructions, which when executed by a data processing system, cause the data processing system to perform a method of identifying tags, the method comprising: transmitting the identification data to the plurality of tags, the identification data specifying at least a portion of an identification code, wherein the identification data comprises a first set of commands of a first type to address tags in a first group of tags which are in a first state, wherein a first command in the first set of commands of the first type causes addressed tags to be placed in a second group of tags, which are in a second state, which are addressable by a second set of commands of a second type, wherein the first group of tags do not respond to the second set of commands of the second type, and wherein the identification data further comprises the second set of commands of the second type to identify tags in the second group of tags in the second state, wherein a second command in the second set of commands of the second type causes addressed tags to be placed in the first group of tags which respond to the first set of commands of the first type and wherein the second group of tags do not respond to the first set of commands of the first type; receiving at least one response from the plurality of tags; and identifying the at least one response from the plurality of tags.
 36. A machine readable medium containing executable computer programming instructions, which when executed by a data processing system, cause the data processing system to perform a method in a tag, the method comprising: receiving at a tag in a first state, a first set of commands to identify the tag in the first state, wherein, as a result of being identified, the tag in the first state is placed in a second state; and receiving at the tag in the second state, a second set of commands to identify the tag in the second state, wherein, as a result of being identified, the tag in the second state is placed in the first state.
 37. A machine readable medium containing executable computer programming instructions, which when executed by a data processing system, cause the data processing system to perform a method in a tag having an identification code, the method comprising: receiving a radio frequency signal from a reader, the radio frequency signal being modulated with interrogation data representing at least a portion of an identification code, wherein the interrogation data comprises a first set of commands of a first type to identify tags in a first group of tags which are in a first state, wherein a first command in the first set of commands of the first type causes addressed tags to be placed in a second group of tags, which are in a second state, which are addressable by a second set of commands of a second type, wherein the first group of tags do not respond to the second set of commands of the second type; and wherein the interrogation data further comprises the second set of commands of the second type to identify tags in the second group of tags in the second state, wherein a second command in the second set of commands of the second type causes addressed tags to be placed in the first group of tags which respond to the first set of commands of the first type and wherein the second group of tags do not respond to the first set of commands of the first type; detecting the interrogation data representing at least a portion of an identification code from the reader; processing the interrogation data to determine if the interrogation data matches at least a portion of the identification code imbedded in the memory, wherein the processing generates a response code; modulating the response code onto a radio frequency signal which may be transmitted to the reader; and transmitting the response code to the reader. 